When you can no longer keep control of the algae that has grown into the pits of the plaster. When you see the gray gunite shell show through thin plaster. When plaster has many pop-ups. When the pool surface is rough to walk on, snags bathing suits, or pool cleaner parts seem to be wearing down in a short time (tires, wear rings, hoses, etc.). When plaster has blue/green or orangish stains from high amounts of metals in water.