How often does a snail change shells?

Snails don't change their shells. Unlike crustaceans, molluscs like snails keep the same shell throughout their lifetime. The shell is formed from secretions from the snails mantle, the flesh under the shell, and some snails (such as the giant African snails kept as pets) can be seen running their mouths along the edge of their shells, to build up another layer on the shells lip.

The stripes that make up the pattern of a snails shell are the result of different compositions of shell material at different times.