It depends. In low pressure pistol rounds like .38 Special., .45ACP, .45 Colt, and .44 Special you can wait until cracks start appearing in the case mouths. I've loaded cases with pretty big cracks and still fired 'em one last time. Just remember these are low pressure rounds not 9mm or magnums.
It's not the number of times you fire the brass as much as it is how much you work it. Working the brass: sizing, belling, crimping, depriming and priming, makes the metal brittle and more likely to crack or tear. The more you work your brass the faster it will wear out. So bell the case-mouths as little as possible and don't over crimp. I find it helps to chamfer the case-mouths as chamfered brass needs less belling. You can also anneal the longer revolver cartridges to re-soften the metal. Of course heating up cartridge mouths with a propane torch (while they're in a pan of shallow water) seems like overkill to me.
/b