Do stamps ever expire?

Tiff: Jo.. have you sent out your letters yet?
Jo: not yet
Tiff: WHAT?!?!? you have not sent out your letters? u like bought ur stamps last week.. it might expire..
Jo: Huh? really meh? Eh.. how.. when will my stamps expire?
Tiff: No they wont expire..
Jo: R u sure? eh tell me leh r u sure they wont expire..