Hair has a capacity to grow one-half inch in a month, which is approximately 6 inches in a year. You can find out how long your hair would be by multiplying this with your age. However, there are other factors that may determine your hair growth rate including your diet and hair breakage. Check out http://www.ehow.com/how-does_5199805_much-hair-grow-month_.html.
technically hair grows .5 inches a month... so .. theoretically you would time .5 with however many months old you are and that would give you the answer. BUT there are other factors involved, one is that the ends get brittle and break off, the other is that we are constantly losing older hair and regrowing it.... so you could only guesstimate at best unless you did extensive research and math XD
Aside the answers listed, you shed hair on a daily basis, so technically there is no way of knowing how "a hair" would grow. However, if you ask a general question as to long a person's hair can go without cutting it, I would refer you to the Guinness World Record book for the longest hair. I'd find the link, but I'd lose my answer, crazy phone.