Welcome!  You ask a very common question.  It is hard to tell you exactly how long it will take, but you can estimate the time by determining the distance from your current length to the desired length.  The average person's hair grows at 1/2" per month, so you can take the number of inches and multiply by 2 which will give you the approximate number of months.

I will warn you, however, that it is not uncommon for hair to "rest" periodically.  This would be a period of time of a month to 6 months where the hair slows or stops growing.  This seems to happen typically as the hair starts to reach the longer lengths such as waist.   

You don't mention your age, but that can also be a factor.  Your hair grows at its fastest rates from 16 to 25 years of age.  It then starts to gradually slow down.   

Try measuring the length of your hair and recording it each month on the same day.  This will give you a better means of estimating your hair growth.   

Come join in the fun on the boards.  We have a topic specifically for recording your hair growth measurements as well as an area where you can create your very own journal.

Hope to see you around more!