I agree with the above posters. You also have to account for the damage that comes along with rough brushing or our hair snagging on things.
I've never bought into the whole Black hair won't grow/grows slowly thing, unless there is a medical reason which is really about the medical reason and not race.
Also, there is a reason why a lot of posters advocate going to some place like the Hair Cuttery for a trim. When people talk their friend of another race growing their hair from this length to that length, I always wonder how short their friend cut their hair.
My friend donates her hair to locks-of-love about every 2.5 years. She got that Kate Middleton hair. She grows it out from about shoulder length to about waist length in 2.5 years, but she doesn't cut it to right at the top of her shoulders. It takes her about 2.5 years to get about 14 inches. That's normal. I used to think that her hair grew fast, but the last time she was kinda complaining about how long it was taking her to get hair hair back to the length she likes (MBL/WL). I think it was because she cut at around 2 years up to what I consider to be neck length (not bottom of ear length), but donated the same length of hair. It took her almost 3 years to get back to where she wanted to be.
Again, that is normal.
Also on me, shoulder length is about 10 inches and waist length is about 22 - 24, so yeah, it will take about 2.5 for me to get to waist length.