Between 6 months and a year old, female boxer dogs experience their first heat cycle. It can start as early as 4 months or as late as 15 months. If a female boxer doesn't have her first heat by age 2, talk to the vet. Since a boxer's heat cycle happens when they're still a puppy, they can become pregnant young. It lasts 1 to 3 weeks and occurs about twice a year. Every dog is different, but most follow a pattern after 3 cycles. Unlike humans, dogs can be in heat their whole lives. Spaying is usually advised, even before the first cycle, to keep them healthy and happy.