How long will it take to grow my hair 4 inches.?

My hair won't seem to grow past my shoulders, my hair was perfectly straight and now its starting to go past my shoulders its just going all flicking and wavy, but it dosnt seem to be growing much, definitely not half an inch a month anyway. And you may say 'it is growing, just its growing out wards because the ends of your hair is flicky' it actually isnt, im sure. Im a teen and in my whole life i have never had hair past my shoulders, it just wont go there.It has always just sat on my shoulders. I dont use heat, i always take a shower and towel dry it a little so its just damp then let it dry naturally, i mean im sure its not good blowing hot air on your hair, I never really straighten it, maybe once a month, someone on here said 'if you have straightened your hair atall in the past then it may NEVER GROW or if it does only grow a tiny bit' and im sure thats not true. So i avoid heat so instead of straitening my hair i just tie it up so you cant see all the flicks. but im sick of my just above shoulder length hair, that i cant do much with.