Depends on how heavy your strings are, and how strong your fingers are. I tend to use 12s on my acoustics, and I reckon on a 2-semitone bend being about the max. My electric runs 11s, and a 3-semitone bend is about the limit for that. If you use very light strings (9s or 10s) then 2 or 3 semitones is easy - the problem is controlling it so that you do small tasteful bends instead of some kind of extreme wide hard-rock bends!

For unison bends, I find 2 semitones is usually sufficient - unison bends are more about where you land than where you start.

Graham.