<p>
It’s simple: people tend to forget things that are not reviewed or put into action.</p>
<p>Coming back to material after a hiatus will show how rusty you are, but it is not difficult to gain everything back. It’s not like learning all over again. So, if you forget things after a year, just look over notes and you will probably see that you didn’t really forget it after all.</p>
<p>This happened to me. A friend needed help with managerial accounting, but I had not been exposed to it for more than a year. I thought it would be difficult, but after 15-20 minutes, things started to come back to me. However, this was only possible with things I had actually learned and understood in the first place.</p>
<p>In short, just go back to your notes/papers/books from your past classes, if available, every now and then, and you should be fine. It’s actually quick and easy since it’s just review, not learning. It’ll be very quick the 20+ time around.</p>