DOG FAQ - How can I recognize when my female boxer dog shows signs of being in heat?

You can note that your boxer vulva dilates. This is the first phase, known at the beginning of heat, and lasts between six and nine days. She'll be attracted by males, but will probably refuse any intent of mating. This phase can last between four and thirteen days, with nine days as average. It's not strange if it lasts around 27 days. Don't be surprised if your female boxer dog urinates with greater frequency during this period, it's normal.

Boxer - Dog Lover