My films tend to be pretty dialogue driven, and, to be honest, I expect, though don't necessarily require, my actors to have memorised their lines.
If I'm shooting a short, then I think a day is more than enough time to memorise lines. A 6-page script, for example, with 3 characters, say, is going to me about 20 lines per character, at most. That can easily be memorised by actors. That said, I am more than happy to prompt actors and allow them to fudge their lines on-set: sometimes I prefer a flunked version of my line to the actual version.
In London, at least, a lot of the actors who come for a day or so on a film, get their primary acting experience from theatre (and pub theatre) so perhaps that accounts for a greater expectation to have learned your lines. If in doubt, ask your actors to learn them: I've had an actor learn 5 pages of dialogue on the train to the location, so no time is really too short for a reasonably smart actor. But, on the whole, I agree with the others. Unless the lack of memorisation is seriously disrupting your shoot schedule, play it scene-by-scene, page-by-page.