They can vary quite a bit, depending on the breeding. Look at the AKC standard and you will see that female ideal never hits very high. I would just go by how she looks, hips and ribs.
For example, my last foster was a 35 pound female at six months. Just by her frame, she will be on the small side of the AKC standard. Previous to her I had a 9 month old male who weighed almost 70 pounds and was over 25 inches tall. Obviously VERY poorly bred. In a months or so he will exceed the suggested perimeters for an adult male. He actually looks like he has great dane in him. He had papers - like that means anything.
It is hard to compare your boxer to others to determine your dog's ideal weight. when I got my puppy, I read weights on here of other people's boxers - I was stunned how huge their dogs were. In Florida there seems to be a trend with backyard breeders to try to breed bigger boxers like they are better than the standard. Tulley and I went to a dog park one day and encountered a 7 month old boxer who was HUGE(scary huge). He also had a very long nose, etc. Well, the lady was beaming and proud. She told me the "breeders" breed some of the largest boxers around. Everyone tells her that she should breed the dog. I honestly was dumbfounded.
And of course some puppys shoot up, and others beef up quicker. Keep feeding quality kibble and watch her ribs. It is no easy task fattening up a healthy, active boxer pup! They burn the calories like no human ever could!:p