How long does it take for a cat to get pregnant?

I have a 6 month old female kitten and two male kittens. We've been keeping them separated until their spay/neuter surgeries. Just a few minutes ago the female got out of the room we had her in the the male started humping her. We separated them. She is getting spayed on Thursday (and the boys are getting neutered). Is it anything to worry about or will the spay go smoothly?