It's different for everyone, depending on one's general look, lifestyle, facial structure, type of curls, etc.
For some folks, the hair might look great at a particular length, but it is too much hassle to take care of, or she might not mind the maintenance, but the look isn't right for her. As the saying goes, YMMV.
But one thing to keep in mind for us curlies, depending on the type of curl and the thickness(amount)and texture(fine, medium, or coarse), the "right" length may be where your curls show in all their glory. Some folks lose their curl when it gets below shoulder-length, for some it gets curlier the longer it gets. On the other hand, someone with very heavy hair that has a looser curl pattern to begin with might be at their curliest length above the shoulders and layered well(good cut, not just some chop-job by someone who doesn't know squat about curly hair!).
health-conscious, visionary artist in transition