How long will it take to grow my hair to waist length?

My hair is shoulder length now, i flat iron it with an expensive flat iron that puts keratin (the building blocks of hair) in my hair every other day so my hair is smooth and healthy but it does get alot of heat I am 14 and my hair is naturally black. idk if that helps. it was a bob at jawline and it took 5 months to get down just past my shoulders. any ideas? an estimate would be wonderfully helpful. also does trimming your hair really help it grow faster? if so how ? (= thanks for the help