Used of an activity or experience, the phrase like watching paint dry, or as —— as watching paint dry, means extremely dull.

It seems to be of American-English origin. The earliest instance that I have found is from ‘The Shrike’ Offered at Horseshoe, published in the Los Angeles Times (Los Angeles, California) of 19^th May 1959:

Sitting in attendance at the Horseshoe Stage Theater presentation of Joseph Kramm’s “The Shrike” is as exciting as watching paint dry. But for the presence of Kevin Hagen in the lead role of Jim Downs, the experience would be almost intolerable.

In an article titled A Growing Gap, published in the Asheville Citizen-Times (Asheville, North Carolina) of 3^rd September 1967, Bob Terrell, Sports Director, described how Eddie Stanky (1915-99), then manager of the Chicago White Sox, took “hard slaps at the press”:

Stanky’s beef is that he doesn’t like the phrases some writers employ in describing his weak-hitting White Sox: “Losing to Chicago is like drowning in three inches of water.” “Getting beat by Chicago is like being whipped by your baby sister.” “Seeing the White Sox score a run is like watching paint dry.” “The batting averages of most of the Chicago hitters read like communter [sic] schedules between 1:30 and 2:30.”

‘Tabard’ punned on the phrase in his column Chit Chat, published in The Stage (London) of 15^th December 1988:

This season’s moving tale

Forget the Regent Street illuminations and thuggish Santas menacing you with charity tins every ten yards, the real sign that Christmas is upon us comes when Luke Rittner talks about moving.
I am sure that there must be instalments in the epic saga of the Arts Council’s proposed move from the faded splendour of 105 Piccadilly at less than annual intervals. You just wouldn’t know it.
Each pre-Christmas secretary general’s briefing has brought a fresh excitement along the lines of “The Arts Council is still deeply committed to moving but hasn’t the faintest idea where to go, except that it will be somewhere in London . . . probably.” This year the tension was palpable. The Arts Council is, repeat, is, currently, now, actually, at this moment in time negotiating the purchase of a new headquarters.
There could be an announcement within a few months but, then again, there probably won’t be because property deals usually fall through anyway.
Council staff who find all this as gripping as watching paint dry have a new diversion – watching paint dry. Even though it is planning to be out within two years, the council has been caught by the clause in its lease which says it has to tart up the facade every so often.
Anyone who has ever been responsible for property will note the restraint in Luke Rittner’s description of the £90,000 bill as “very galling.”