I think it says that he can be very patient. Personally, I wouldn't have the patience to wait for my hair to grow long. Also, there's that awkward inbetween phase where it doesn't look as good as if it were just short or completely grown out. So, it may also indicate some level of commitment or something along those lines.
if its straight long hair i suggest he is just an avid rocker or biker but if its just long and messy to the point it goes past the cute stage for females, i just guess they are unhygienic and given up or something.