If you use the search function at the top of any page for "snail acclimation", you will find a lot of threads on the subject.
When acclimating snails (except nerites - I will use Bang Guy's method for these the next time I try) I float the bag for maybe 30 mins for temp, then discard a shot glass of bag water and replace with one of tank water every 15-20 mins for maybe 3 hours. Snails (and inverts in general) are much less tolerant of sudden changes in water parameters than fish are.