Hermit crabs

[Serafin]

I have 3 red legged hermit crabs on my tank atm, all have shells around 1'' long with openings around 0.8-1 cm wide. How long will it be before I have to buy bigger shells like 1-2cm wide, or shall I just buy some similar sized shells for now? As I'm finding it difficult to find out just how quick they will out grow there current sized shells.
Is there anyway to tell if they have outgrown there current shell?

Any help will be appreciated.