I've been to cinemas in a few places around the UK, and it's been The Twenty[0] - 10 mins of ads, then 10 mins of trailers, from the time the show is scheduled to start, before the movie - for well over a decade. However, of late at my local multiplex, it's definitely stretched out to more like 15/15, with 30 mins between the posted start of the show, and the start of the actual movie.
[0] http://en.wikipedia.org/wiki/The_Twenty