Scalp hair grows at about one millimeter every 3 days, on average. It's a bit faster in kids than adults and faster in women than men. So a 10 year old girl's hair might grow more like one millimeter every two days. There are about 25 millimeters in an inch, so on average it would take about 75 days--or two and a half months--for your hair to grow an inch. Of course this is average value. Some people's hair grows much faster and others have much slower growing hair. If it can grow an inch in about 2.5 months, then it should grow about 5 inches or about 12cm in one year.