The way I see memory, when you learn a fact you are most likely to forget it (if you forget it) within a day or two, if not sooner. If it makes it past that point, prospects for retention are much better.
So the trick is to develop a system in which you reinforce the facts within several hours, and again within a few days. By extention, it would be beneficial to reevaluate retention at a few weeks, few months...