How much time is required to relearn a subject after x years? I'm searching for research answering or giving any form of insight to this question:
How long does it take a person to relearn something (any type of learning) when relearning happens x, x+1, x+2...x+n years after initial learning?

I've found some research by Ebbinghaus, but it concerns relearning after 24 hours. The variable in his research is number of repetetion (and not time between initial learning and relearning) which isn't what I'm looking for either.

In particular, I'm trying to figure out the effectiveness of students studying a subject in college, and how long students will need to relearn a subject, if they previously learnt it 5, 10 or 15 years previously.

References


Ebbinghaus, H. (1885). Memory: A Contribution to Experimental Psychology. (H.A. Ruger &amp; C.E. Bussenius, Trans.). New York: Teachers College, Columbia University.
Suppose a person learns a subject in college and waits for 10 years before learning it again. An exam is given one week after the person relearns the subject. So in this case, the ISI (inter-study interval) is very long compared to the RI (retention interval). The person will definitely forget some of the material after the 10 years. So how long they would need to restudy depends on how much they have forgotten. In this paper, Hal Pashler and others develop a model that predicts the spacing function based on the forgetting function for fixed retention intervals. The forgetting function depends on the type of material being studied and other variables. All other things being equal, it seems that testing/examinations would be one of the more efficient ways of relearning the material.

References

Mozer, M. C., Pashler, H., Cepeda, N., Lindsey, R., &amp; Vul, E. (2009). Predicting the optimal spacing of study: A multiscale context model of memory. Advances in neural information processing systems, 22, 1321-1329. PDF