there are a few factors that come in to play with this.... one is how much sleep you get.... hair grows fastest in your sleep so if you get a lot of sleep it will grow back pretty fast.... also, you have to think of the elements of hair; notably vitamin E.... the more elements of hair that you consume and have in your system, the faster your hair will grow.... metabolism also plays a factor....
the same goes with a person's nails.... i always tell my girlfriends who complain about having weak nails to consume more gelatin to make them stronger because gelatin is actually made from the same compounds that are in nails.... if you have weak or brittle nails, eat jello every day for a month and the results will amaze you....