8 to 10 weeks
https://animals.mom.com/how-soon-after-a-litter-can-you-spay-a-cat-12620239.html
Spaying During Weaning
You’ll be able to start weaning your fluffy pals around 4 weeks of age, the ASPCA says. During this period, you can separate the kitties from their mama for a few hours at a time. Feed them gruel, kitten food heavily soaked in water or kitten milk replacer, for four to six weeks -- between about ages 4 weeks and 10 weeks -- until they are no longer nursing. This weaning stage is the soonest you should spay Mitsy after she gives birth. She can still produce milk for her babies when they need it, but they’ll be content being away from her for periods of time as they learn to eat and take care of themselves.