Usually a couple weeks is a good time line, but is can depend on how much old food you have left and what your rabbit tolerates. If you don't have a lot of food to transition with, then you have to go faster. If your rabbit doens't tolerate change well, then you need to go slower.
I usually do a 50/50 mix and feed that for a while, usually until I run out of the old food. My rabbits tend to transition well and I haven't had many problems with this. Some don't like going to a new food, so may resist more than others. I haven't had digestive problems though.