Since you're checking for incipient case separation and not finding any and you're not loading at maximum pressures, I'd keep using them until you see the first sight of incipient case separation before chucking them.
If you're shooting them in the same rifle and neck sizing, you're probably not working the brass very much.
I have 100 cases for each game rifle and either 200 or 300 for each colony varmint rifle that I load until I see the first sign of incipient case separation and then I chuck them and start over.
