How common is it for somebody to get vaginal atrophy?

Common at menopause. The vagina requires estrogen to stay healthy. At menopause the estrogen levels decrease and vaginal atrophy begins to develop. It is easily treated with estrogen replacement - sometimes with oral medication and sometimes with vaginal medication, and sometimes with both routes.
Very common. After menopause, if not using estrogen supplements. Very uncommon before menopause. Occasionally seen in young patients taking birth control pills.