Your hair grows about a half inch per month or half a millimeter a day.  This means that hair grows about 6 inches per year. Genetics, hormones and age are some factors that affect how fast hair grows.
Ethnicity also affects the rate of hair growth.  A study conducted in 2005 indicated that the hair of Asian females grew almost 6 inches per year, while the hair of their Caucasian counterparts grew a little more than 5 inches per year. The hair of African females only grew 4 inches per year.
The seasons may also have an impact on how fast hair grows.  Hair appears to grow faster in the summer, probably due to increased exposure to UV radiation, hormone shifts and increased intake of vitamins and minerals present in fresh produce available during that season. The increased hair growth is minimal, however, to the point that it is barely noticeable.
Still, changing your diet to include foods that stimulate hair growth can't hurt.  Zinc and iron, found in meats stimulate the growth of hair follicles. Vitamin D stimulates hair growth as well. Spending a few minutes each day in the sun or taking a supplement gives you ample supplies of this nutrient. Protein helps build cells and healthy hair, and omega 3 fatty acids help condition your hair.