Many shelters spay and neuter as early as 6 weeks. The rescue/shelter Where I volunteer waits until they are 4 months old. Most vets agree that between 4 & 6 months is best.

Do NOT wait until she has had a heat. A dog does not need to go through a heat period. It is easier on the dog to have it done early. Keeping a female away from all males during a heat is extremely difficult. Males have been known to break windows to get to their lady love. And a quick trip outside to potty has been known to turn very young dogs into mothers.

A young, healthy dog will be less active for a couple of days after surgery. As long as the stitches aren't licked out, the recovery time is minimal. Older dogs take a couple of days longer.

It's a good thing for both you and your dog to do it early.