How long after cat has babies can she be spayed?

Just after weaning. As someone mentioned, that's about 8 weeks - but remember to NOT let her out during that time, since she can get pregnant just two weeks after delivery again, and you don't want that.

As soon as the kittens are weaned, at 8-12 weeks.

after the kittens are weaned. at 9 to 12 weeks

When the kittens are weaned. Preferably when they are 10-12 weeks old.