How long is a piece of string?
It depends how it's been looked after and the quality when new.
A good make well looked after could be good for well over 10 years.
A cheapy left out in the sun and covered in saltwater could be ruined in a couple of years.
With pvc boats, anything over 5 years and I would be inspecting the seams very closely for separation as the glue can start to go off then.
__________________
|