Is bulletproof glass bulletproof forever, or does it eventually wear out?

When I say forever, I am referring to it?s durability in using it repeatedly. Since it is bulletproof, does that mean it is *always* capable of stopping/deflecting bullets, or does it mean it is like a one time thing? For example, say a police car that has bulletproof glass windows gets caught in a shoot out. If the car windows get shot six times, does it still hold the same durability as it had before it was shot, or can it, (after a lot of hits) eventually break?