Some will make 30 inches, some slow growers will make 15 inches at 1/4 an inch a month in five years. Now in five hears how much have they cut? And did they have any setbacks. I alone have had set backs from Grave's disease and then when I went hypo thyroid from RAI, and then from anemia, and again from severe anemia this year when my hair decided not to grow at all (so severely anemic that doctors wanted me to get a blood transfusion. It was because of my fibroids). So now if you then want to add slow growth moments, or other set backs (I had a bad cut at one time too where someone took sheers to my hair. And I had to take two years to even all of that out).
So you really have to factor in the unexpected. I'd say getting to mid back or waist is a great accomplishment. More if you have limited set backs or illness in that time.