There are several signs that can indicate you need to replaster your pool but these are the most common:
It is harder to keep algae from growing into the pits of the plaster.
The plaster is heavily stained.
You can start to see the pool shell from the thinning plaster.
The plaster is popping up in places.
The pool’s surface is rough and scratches the skin, bathing suits, or pool cleaner equipment.