Very interesting research that says “increasing the time between the study sessions reduced the rate of forgetting. This reduction in forgetting was very large.” They go on to give some recommendations and other practical information. This is a “must read”.

practice makes perfect" and "timing is everything…Proper spacing of lessons, the researchers report, can dramatically enhance learning. And larger gaps between study sessions result in better recall of facts.

Improving Long-term Learning Through Spacing Of Lessons