Brake cables are cheap and easy to replace. Once you think you need to change them - then you need to change them.
I guess the definitive answer is when any strands break at either the ferrule in the lever or at the adjustment nut on the calliper. This is not the same as just getting a bit splayed and flattened. Make sure you use one of those soft metal end caps on the cut cable as they do keep the strands under better control beneath the adjustment nut.