Anything between a month and a couple of years, depending on how invested I am, the amount of source material on offer, and how large and active the fandom is: larger fandoms tend to hold my interest for longer because there is more fic and more new activity in general. My average length of stay in one fandom is probably five months, thinking about it.

I do have some fandoms that I never totally fall out of love with: they stay in the background for months or years, then suddenly something will re-trigger my interest in them and they will erupt back into my life. Kinda like herpes.