Please tell me that Wookieepedia have their facts wrong and that the Rebellion was longer than that. My whole plot rests on a timeline spanning, at least, a decade. If you confirm that the Rebellion was indeed just seven years, I'm going to have to do some rethinking.
Ah, that's an easy answer! It's whenever you want! Your story best served by small pockets of rebellion slowly coalescing into one unified fighting force halfway between EIII and EIV? Then it is so. You like the Force Unleashed? Then it was never junked. No rebellion at all? Fine! Canon at your table is whatever you say it is, Lucasfilm and Disney and Wookieepedia be dammned.
I'm going to like that comment simply because that is my attitude towards gaming. You should have seen me when I used to play with Star Wars toys as a boy. That was most definitely non-canon but I was happy.
Only reason I wanted to stick to canon was to add that sense of....ahem.....realism to my games. But I think I'll just not stress and do my own Rebellion. They can join the Alliance later in the story. Gives me about 19 BBY to 4 ABY, hence 24 years to play with.