I'd say if you want to make it last learn how to apply it.

Music theory is one of these things I learned multiple times (even as a minor in University) and forgot multiple times.

The theory I learned, though, was never really relevant to my artistic work, whether it was playing guitar music or making electronic music. Whenever I tried writing something based on the theory I learned, I found it to be super boring. Thus, to this day, music theory doesn't make a lot of "sense" to me.

Thus, I'd say if you want to learn music theory, try to make it part of a creative process. Learn something new and try to write a little piece based on that, and so on. That way, it gives you creative tools instead of just being boring.