And for anyone who's interested, I've done the math to see just how long Rapunzel's hair would have to grow per month over 18 years to get it to 70 feet. On average, hair grows half an inch every 2 months. (ON AVERAGE. Personally, my hair grows about an inch every month; I'm a freak of nature.) If the average person goes 18 years without cutting their hair, it would get to 4 and a half feet.
Rapunzel's hair would have to grow about four inches per month to get to seventy feet by the time she turns eighteen. (Actually, it'd be a little over seventy feet at four inches a month.) That's about an inch a week.