Originally posted by NastyInThePasty:
Well, we at least know that Amy was growing her hair out... 
Yup, though I like to think there may have been some lovely hair extensions in there. It usually takes longer than two years for shoulder length hair to grow long enough to be hip length, even for people whose hair grows fast. Plus Amy does not seem patient enough to me to allow her hair to grow out without some sort of enhancements that would make it easier and prettier. She seems to me she would always want to reflect the latest styles so letting one's hair grow out naturally would not let her be able to change it all the time to reflect the latest styles.
Then again, maybe they have some super duper hair growth product in the 31st century where you can let it grow to your hips in 5 minutes.