Pretty much as soon as the kittens are born as I understand. It's more to do with what time of year it is (amount of hours of light etc)
Watch out because they can get themselves in the club again even if they are nursing kittens! I'm pretty sure I've got that right. Vets usually like the mother cat to be a couple of weeks clear of feeding the kittens before they will spay them...I think it's to ensure that her hormones are all level again...so probably best to try and keep her in, easier said than done I know.