Spaced repetition is good for learning, but you need to be using it all along. You are basically "reviewing" the material, like for an exam, but in smaller doses, continuously. It does not help you cram if you haven't been keeping up. The "spaced" part is based on refreshing your knowledge just as it is fading from the previous repetition. As time and repetitions pass, the repetitions can get farther apart, because it is increasingly "fixed" in your memory.

It depends on # of repetitions and whatnot. http://www.gwern.net/Mnemosyne.html#fn3 :

"With himself as the subject, Ebbinghaus found that for a single 12-syllable series, 68 immediately successive repetitions had the effect of making possible an errorless recital after seven additional repetitions on the following day. However, the same effect was achieved by only 38 distributed repetitions spread over 3 days"

This wouldn't work with the SM-2 algorithm that Anki and Mnemosyne use; you would grade the series flashcard highly after a few repetitions and then it'd get pushed out a few days. So you'd have to lie to it - the SM-2 algorithm is meant to use as few repetitions as possible over the long term. It optimizes for retention over years, not days.

Anki does have a 'cram mode'. I don't know how it works or whether it trades efficiency (in # of repetitions) for retention in the short term (next few days).