
I have had my girls spayed 12 weeks after they had their last litters, which would be some 21 weeks from their last season. Any later and they may be too close to coming in season again.
I prefer not to wait another season as mien is a double coated breed that looses every stitch of coat after pups and won't be in full coat until pups are about 6 months, and when you spay them they often have another heavy moult about 8 weeks later, so would have to wait another few months for them to be showable.
When spayed at 12 weeks after the pups they only have the one moult, and then after that moult as usual.