Zero to four weeks!
My vet said I have to bring the mother cat for spaying five weeks after the
kittens are born.
It won't affect the milk production, and there have been cases where there
were already new babies in the womb, so she stated.

Carola

I had a cat give birth and when her kittens were 9 weeks old, she went into
heat again. I was told you can spay a cat and she can continue to nurse her
kittens. I'd get her spayed sooner rather than later. If she does get pregnant
again, you can still spay her if she's pregnant, it just costs more because
it's a little more difficult.