Typical human hair consists of the hair shaft, which sticks out from the skin, and the root, which is sunk in a follicle, or pit, beneath the skin's surface. Except for a few growing cells at the base of the root, the hair is dead tissue and made up of keratin and related proteins. The hair follicle is a tube-like pocket of the epidermis that encloses a small section of the dermis at its base. Cells divide rapidly at the base of the follicle to form human hair. As the cells push upward from the follicle's base, they harden and undergo pigmentation. And, voilà, you have hair.
OK, so this process happens over and over again. You're probably thinking that if you never cut your hair, it would just grow and grow and who knows how long it would be. It's not that simple. Your hair actually doesn't just grow and grow. At any given time, some of your roots (about 15 percent or so) are actually on a growth hiatus. For about three months, there's no activity in those follicles. Which means no hair growth at that particular site on your scalp. You lose hair every day, either due to normal shedding or due to damage. On average, you lose anywhere from 50 to 100 hairs every day.
The hairs on our heads grow about half an inch per month and have an average life of two to six years. From this, you can figure that an average person's hair should grow no longer than 3 feet or so.
.5 inches x 12 months x 6 years = 36 inches or 3 feet
Considering some people's hair might grow a little faster and could possibly remain attached to their head for a longer period of time, it's possible that a person's hair might grow even longer than that, say about 5 feet. But that would certainly be less common. So, it's highly unlikely that you would resemble Cousin It from the Addams Family.
It's interesting to note, however, that there have been documented cases in the Guinness Book of World Records citing considerably longer heads of hair. In 1993, Dian Witt of Massachusetts reportedly had locks measuring 12 feet 8 inches in length, and in 1994, Mata Jagdamba of India sported tresses 13 feet 10 ½ inches long.