My Boxer is in heat?

The average is three weeks (21 days). Some bitches only go into heat for two weeks and some can go as long as four (but anymore can be hormone problems and you should take her to the vet to get checked).

As for keeping her diapers on... there's really no way of making her like them. Every time that you see that she's fussing with them or chewing on them, tell her "No" and turn her attention to something else... perhaps a toy, food, or treat.

It's not a period. It's a heat cycle. She probably won't leave the diapers on no matter what you try. She will be in heat 3 weeks. Keep her away from other dogs during the second week, so there is no accidental mating. If your dead set on breeding find a reputable breeder that will mentor you. Please do your homework. Do you have the money and time to spend on her & her puppies ? Can you afford gentic testing ? Dogs can even contract STDS ( herpes is one of them). Sometimes things go wrong can you afford to lose money ? Vet work isn't cheap.Can you keep the puppies til suitable homes can be found ? Are you willing to refund money if puppy has a genetic disease ? Are you willing to take back one of your dogs/puppies if their family can no longer care for it ? Breeding dogs is a serious business, your dealing with living creatures. If you breed you need to be responsible for every puppy you raise.