Eh, it probably depends. I mean, the Rebel Alliance only just formally announced its existence as an organization, which could speed things up a lot.

[ But Kanan is a Jedi, and part of being a Jedi is being able to see multiple possible paths for the future. ]

It's possible the Empire could stomp it out completely before it really gets going, in which case it'll be quick. But if they manage to get some hits in, do something big and symbolic to attract more recruits and defectors - it could drag on for a while.