When asked for a simple heuristic in how to use the spacing effect–the
finding that repetitions spaced in time are more effective in
supporting remembering than repetitions squished narrowly in time–I've
often told people that the ideal spacing interval is one that will equal
the retention interval one desires. If you want you learners to
remember for a month, give them one-month spaced repetitions. If you
can't do that, longer is better, and there seems to be something magical
about repeating something overnight.

But new research suggests that even short spacings of only
half-a-minute or so can have lasting benefits over non-spaced
repetitions.

Citation:

Rawson, K. A., & Dunlosky, J. (2012). Relearning Attenuates the Benefits and Costs of Spacing. Journal of Experimental Psychology: General. Advance online publication.