From a few day up to 3 weeks. The vet can still spay her while she is in heat. I had held my Allie back until Bailey was ready so they could be in recovery and have to stay quiet at the same time. So my vet said that if Allie went into heat sooner, I could still bring her in to put an end to it. And if you don't spay her right away she can go back into heat over and over until you do.