The hair actually grows back a little bit thinner each time. I know people who started waxing at 14/15 and by 25 they didn't need to wax more than once a year.
It depends on the person as to how long it takes for the hair to grow back. Mine takes 3 weeks, and I usually have to wax once/twice a month. Other friends of mine only need to wax once every 2-3 months.