According to the author of SuperMemo, the lifetime cost of learning a particular fact for life, in a perfectly structured spaced repetition system, is between 3

According to the author of SuperMemo, the lifetime cost of learning a particular fact for life, in a perfectly structured spaced repetition system, is between 30 seconds and 5 minutes[1]. Thus, you should never memorize a fact that will come up so rarely and is trivial enough to look up that it doesn't exceed that lifetime threshold. The vast majority of state and national capitals fall into the "not worth it" category. Also, most students don't have nearly the efficiency of learning that a spaced repetition system affords (Especially Wozniak's highly optimized system wherein he has a database that he habitually has quizzed daily for years), so for them the cost is dramatically higher. 1: http://www.supermemo.com/articles/useless.htm http://www.supermemo.com/articles/useless.htm > lifetime minimum cost of remembering a single well-formulated piece of knowledge can easily be estimated to fall into the range from 30 seconds to 5 minutes (minimum cost is the cost that is achieved using the optimally spaced repetition schedule). This estimation can be used in cost-benefit analysis in deciding which mission-critical pieces of knowledge should be subject to learning based on spaced repetition