Probably up to a week. You may only notice milk leaking for a few days, though. She is at risk of mastitus if the kittens were drinking a lot of milk and then suddenly were lost. You might want to consult a vet about how to look after your feline and keep her healthy through this time.

The highest risk of mastitis (an infection of the milk teats, made more likely by engorgement or sudden cessation of feeding) is during the first few days after the kittens were lost.

Treatment for mastitus is a pretty straightforward course of antibiotics. Well, as straightforward as it ever is getting medication into a cat!!

I suggest you get your cat spayed so that she can't suffer like this, again.