Depends how often I am loading and unloading my gun with the ammo. When the case rims start to get chewed up from cycling or enough rounds start showing signs of setback it's time to switch them out. I always put these rounds aside rather than shoot them so I have a little stash of the expensive SD ammo put aside. Usually about 3 times a year ill switch them out.
I don't rechamber the same round again for my SD ammo. I put a new round in the magazine and rack the slide to chamber a round in the barrel. I then put the chambered round in my range bag and shoot them off on my next trip to the gun range.
You are just wasting your money. Unless the round shows signs of setback there's no reason not to rechamber it a couple more times. Although if I were using a higher pressure round like the .357sig, or better yet a +p variant, I would be a lot more wary of setback.
For those who don't know what bullet setback is, when the round hits the feed ramp while chambering it pushes it back into the casing. Pushed back enough and it increases pressures, and sometimes to dangerous levels. The higher pressure the round the more cautious you should be as it can spike dramatically.