That could be a long time, and she'll be in heat soon and getting pregnant again. I hope you're keeping her inside all the time.
Most vets will tell you they prefer to spay a cat who is not in heat or nursing. That's because the surgery takes a little more work to tie off the engorged blood vessels. It's for the vet's convenience.
I suggest you call back and tell them you'd like her spayed now regardless, and if they refuse, find another vet who will.