I have the hardest time making myself get trims when I'm attempting to grow my hair long for donation. It feels so very counterproductive that I try my hardest to put it off as long as possible, but that isn't always the best solution.
The average rate of hair growth is about a half an inch (1.25cm) per month, and if you get your hair trimmed every 8 weeks, you'll have a total hair growth of about three inches per year. Since that's way too slow for people trying to grow their hair long for donation purposes, I suggest getting your hair trimmed every 12 to 16 weeks. But in order to wait that long between trims, you need to keep your hair in optimal condition. Which mostly means, stay away from unnecessary heat (i.e. blow dryers, flat irons, etc), take a daily multi-vitamin, and substitute conditioner for shampoo every other day.
It's best to recognize early on that not trimming your hair is a losing battle. Your hair will end up dried, fried and split. So get those trims and watch your beautiful healthy hair grow!