We talked a bit about this in my old shipping bag salinity thread.
I generally test the bag water and adjust acclimation accordingly (don't know if I'm really doing any good by that, but it makes me feel better)
Fish from LFS in water roughly the same pH:
~20-30 minute drip *Saxman posted a fish acclimation procedure that I keep in mind now*
Crabs, shrimp etc:
~2 hr float
Snails, except nerites:
~3-4 hr float
Bang Guy posted an acclimation procedure for nerites yesterday that I will use the next time I try.