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 & C.E. Bussenius, Trans.). New York: Teachers College, Columbia University.