I measure my hair every month for 6 months, and then I divide the number for example say I measure my hair and it's 16 inches one month, 16.75 the next month, 17.5 inches, 18.25 inches, 19 inches, 19.75 inches, I would add those numbers up giving me: 107.25 divide that number by 12 to get your yearly growth rate: 107.25/12=8.93 inches a year. Divide that number by 12 which will roughly give you an estimate of how your hair might possibly grow a year: 8.93/12=.74 inches a month, a little more than half an inch a month.
If you keep a journal it will be easy to moniter your progress.