In American English, the phrase French hours has had at least three meanings:

1-: In the text containing the earliest occurrence of French hours that I have found, the phrase denotes, with reference to the French air force in Vietnam, a workday with a long midday break for a substantial meal and a sleep. This text is as follows, from The Evansville Press (Evansville, Indiana) of Saturday 24^th April 1954:

French Certain Our G.I.s Have Come To Stay
By Jim G. Lucas
Scripps-Howard Staff Writer

Tourane, Central Vietnam—The 140 American technicians at this French air base have just worked themselves out of a job.
[…]
These healthy young Americans find inactivity stifling. They are confined to the base and after dark to their compound.
[…]
The French have a beer garden on the post and drink during working hours. Captain Woodward put it off limits for his men. He says beer and airplanes don’t mix.
The French stop work at 11:30 a.m., eat a big noon meal, sleep and come back at 3 p.m. Then they work until 6:30 p.m. The Americans first tried ignoring this. Since April 10, however, they’ve worked French hours. They still think it’s foolishness.

2-: Used of filmmaking in France, the phrase French hours has been used to denote the workday, beginning at noon and without meal break. The earliest occurrences that I have found are as follows:

2.1-: From The New York Times (New York City, New York) of Sunday 11^th November 1956:

PRODUCTION VIEWS ALONG THE SEINE
By Gene Moskowitz

Paris.
Unlike most American film directors who come to Paris, Billy Wilder feels that “Love in the Afternoon,” which he is writing, directing and producing for Allied Artists release, could have been made just as well in Hollywood. […]
[…]
Speaking excellent French and accompanied by a nucleus of technicians who have been with him on all his films, Mr. Wilder is at home. He feels that French workers are as efficient as their Hollywood counterparts. In fact he says that their individual initiative makes studio conditions easier in Paris. He also likes the French hours—noon to 7:30 P.M.
“In Hollywood, with the five-day week and early hours, one can make fifty silly errors before one begins to work,” he said, “but here, at least one is conscious of the mistakes one is making.”

2.2-: From the Hollywood column by Sheilah Graham (Lily Shiel – 1904-1988), published in several U.S. newspapers on Monday 5^th August 1963—for example in the Watertown Daily Times (Watertown, New York); the following is about the Mexican-born U.S. actor Anthony Quinn (1915-2001), who was in France for the filming of Behold a Pale Horse:

Paris, Aug. 5.—[…] Quinn’s house in Versailles, which he is sharing with Jolanda and their infant son, is an hour and a half’s drive from the studio in St. Maurice. But the trio want privacy and to get it, Tony is on the road three hours a day. Not too tough, because of the French hours of studio work—from noon to 7:30.

2.3-: From Gina Lollobrigida: Glamorous Star And Homebody, by Marjory Rutherford, published in The Atlanta Journal and The Atlanta Constitution (Atlanta, Georgia) of Sunday 16^th September 1962:

Gina’s days off are rare when she’s making a movie. “Imperial Venus,” a multimillion-dollar production being filmed jointly by two French and two Italian companies, has an international cast working “on French hours—12 noon to 8 p.m.”

2.4-: From Warning: Skip Lunch For Today, published in the Columbus Evening Dispatch (Columbus, Ohio) of Sunday 19^th February 1967:

In the Paris movie studios filming starts at mid-day under the so-called “French Hours” system. Consequently, actors usually sleep late and have an early lunch before they face the cameras.

2.5-: From Lilli Prefers the Country Life, an interview of the German actress Lilli Palmer (Lilli Marie Peiser – 1914-1986), published in the Boston Herald Traveler (Boston, Massachusetts) of Tuesday 20^th August 1968:

She prefers to work in Paris on motion pictures to any other country.
“The hours are so suitable for the perfect working day—from 11 or 12 noon to seven or eight in the evening,” Lilli said. “Women don’t really look well in the early morning. The best time for photographing them is about 1 p.m.
“So the French hours appeal to me. There is no lunch break and it seems to me that more gets accomplished this way.”

3-: Used of filmmaking in the USA, the phrase French hours denotes a workday without lunch break, during which, instead, food is constantly available.

In this sense, the phrase was apparently first used by the U.S. film director Joel Schumacher (1939-2020) of the filming of his thriller Phone Booth (2002). The following two texts quote him as using French hours:

3.1-: From How many directors can you fit in a Phone Booth?, by Johanna Schneller, published in The Globe and Mail (Toronto, Ontario) of Friday 4^th April 2003:

With a tiny budget—$1.5-million (U.S.), his smallest ever—and a host of extras, Schumacher shot in continuity, 10 days in a row. “We had to stop shooting by 4 p.m., because we lost the light,” he says. “So we would get there at 6 a.m. and roll by 6:45. We did ‘French hours.’ You don’t break for lunch, you just keep passing food all day. Colin’s * confession at the end, that’s the ninth day of shooting—it’s the first take and the only take. He was exhausted and you can see it.”

* The Irish actor Colin Farrell (born 1976) interprets the role of Stuart Shepard in Phone Booth.

3.2-: From Thinking outside the box, by Robert W. Welkos, published in the Los Angeles Times (Los Angeles, California) of Monday 7^th April 2003:

Filmed in only 10 days in December 2000, “Phone Booth” initially had been slotted for release by 20^th Century Fox the following autumn. After the Sept. 11 terrorist attacks, the studio decided to delay the release until November 2002 […].
[…]
Because it was late in the year, darkness descended at 4 p.m., so the cast and crew were asked to shoot “French hours.”
“The whole crew had to vote on it,” Schumacher explained. “You work straight through and pass food around the whole day. You don’t break for lunch.” They shot 10 pages of script a day.

In the following, published in the U.S. business magazine Fast Company on Sunday 1^st August 2004, the U.S. author and entrepreneur Seth Godin (born 1960) examined French hours in terms of “work rules and union policies”:

French Hours
With “French Hours,” you can boost efficiency—and eat all the croissants you want.

[…]
One reason that [Hollywood] movies cost so much is that the very same work rules and union policies that let a film crew run like a well-oiled machine also impose a hefty tax on flexibility. You might want the key grip to help hold the boom, or the lighting team to come in a few minutes early to set something up, but they won’t. They won’t be nasty or intransigent, but they just won’t go the extra mile. The rules are crystal clear.
This applies even to lunch. At a certain time, after a certain number of hours, the set shuts down. Lunch is served. Not just a granola bar. Lunch. It’s astonishing how well fed everyone is on a movie set.
After the mandated lunch break, everyone gets back to work, but the loss of time and momentum is huge. And you can’t make it up by staying late. If you stay even a minute past the cutoff point, everyone gets a significant overtime bonus.
While this rigidity serves a purpose, it underscores a critical thing about work in the movies (and work everywhere else). If we get too comfortable, if everything is business as usual, it can be hard to create something remarkable.
This system is so ingrained and so universal that I was stunned when I heard producer, writer, and director Joel Schumacher talking about “French hours,” likely named for a French filmmaking technique that is partly responsible for the very good—and very cheap—movies that get turned out all the time in France. French hours are simple: If every single person on the set agrees, you can film a movie without lunch breaks. They serve food (just as much, and probably croissants too) all day long. People eat when they can. The production never slows down. Things move quicker, and efficiency is much higher. The shoot changes to an emergency footing, and the team gets it done as quickly as they can—while still maintaining quality.
Schumacher filmed the thriller Phone Booth this way. The entire shoot took 10 days. By contrast, the more average film Almost Famous took 92 days. Not only did the esprit de corps and sense of urgency of French hours allow Schumacher to meet his timetable, but they also produced a better movie. The tension in the performance of every actor is palpable. Nobody looks as though he just had a big plate of ziti.