One thing I've learnt is to look for the topics I keep coming back to again and again. I do sometimes get interested in something for a short period of time and never return to it. But there are a core group of subjects I keep coming back to, even after breaks of a few years.
Of course, sometimes I can't tell if a subject I'm on hiatus from is one I'll ever go back to. But I just run with it. If I never return, that's fine.