Brass is "used up" when it fails - failure could consist of a split in the neck, a crack in the side wall, an incipient head separation, or a primer pocket that is stretched tothe point that it takes little or no pressure to seat a new primer. If any of these conditions is observed, "retire" that case!

I never arbitrarily throw out a cartridge case just because it has been reloaded "X" number of times. How long a case will last depends on how agressively you work the brass, how sloppy your chamber is, how hot a load you use, and perhaps several other factors as well.