Half an inch a month is average, but more or less as suits your purposes is within the range of normalcy.
Be aware that if he does no trimming, he'll very probably have split ends which can make it seem as if the hair has stopped growing when really it's breaking off at about the same rate as growth.
In the hippie era, a lot of guys' hair seemed to stop growing just past their shoulder blades, and this was nearly always the reason. Our son's has stopped there, no seeming growth for three or four years, since he will not let anybody touch it with scissors.
If your ex-Marine uses products to protect and strengthen the hair shaft, and gets trims three or four times a year, his hair will be substantially longer. However, that kind of attention to his hair will also say something about his character and his priorities, so do what fits him.
I would think it's pretty realistic for your character to have hair that reaches the bottom of his shoulder blades but falls well short of his waist.
Maryn, whose hair is down to there
