I think the only part of the film that gives us any indication of how much time passed, is when Jack first talks to the doctor, who tells him he's better off hospitalized rather than going home with someone, and that he will eventually go to where he was when he was catatonic in the past.
But I don't think he had yet gone anywhere else, was still in that first hospital. That seems to me to indicate a few weeks at most.
Another thing: Lydia visited him at least a few times while he was in a coma. When Jack first comes to visit him -- that night when he rants on about if he's going to go after the grail or not -- he stops and pretends to watch TV with other patients while Lydia is in the background having a conversation with a nurse about where a blanket (or something) went, to which the nurse replies that it's being washed because a doctor had a 'hypo moment' or something like that. That Lydia moment kind of indicates to me that at least enough time passed for her to be there a few times, so probably more than just a couple of days.
So it was probably between a week and a month, not much more or less.