It has been found that, within two days, if it isn't reviewed, people remember less than 70% of any subject matter they have studied. By the end of the month, the figure falls to 40%. On the other hand, if it's looked over again within 2 days, then 7, you should be above the 70% level until the 28th day. Another review then should make it remain long-term. In fact, short and frequent bursts of study are more effective than one long one - the brain appears to like short "rests" to assimilate knowledge. Constant reviewing is the key, especially for a short time at the end of each day.
(Source: Ohio State University).