
I have had bitches spayed when their pups have been 11 or 12 weeks old, as long as the milk has dried up, and mine often feed a little until pups are 8 weeks.
Some bitches will come in season as they would have if not bred from so whatever her usual time would have been had she not had a litter.
So a bitch with 6 month cycles could well be back in season when pups are around 15 weeks old.
My own bitches normally are about two weeks longer at least often 4 weeks.
So if your bitch has shortish season intervals I would get her spayed when pups are 11 or 12 weeks if she is dry.