It is dependant on your enviroment. If you get the temps below 60 and keep it dark it will grow. it does not have to be freezing to grow hair. You just have to trick the body into thinking its coming winter.
We have been able to grow hair at the 60 deg. setting on the AC unit with fans going. Key is keeping it cool enough to be comfortable and total darkness for the 12 hours a day they spend in it. Colder would be nice, but would cost $300 for a Cool Bot.