Depends, on how hot your load is, brass brand, and just shear chance. There is not scientific formula or anything specific. brass gets fatigued.

I have +P starline brass and load moderate for my Model 21. I feel safe to 5 times. Thats probably overkill. Makes more sense IMO to buy a bar sto, Jarvis, Wilson Combat or even loan Wolf (but don't know anything about them)