every 8 weeks give or take. you need to get regular trim when you're trying to grow your hair or else your ends will become extremely unhealthy and when you do at last go in for trim, they'll have to chop a lot off. if your hair is really short now, you can probably go 4 months without a trim and not have bad ends. but if your hair is already past your collar, then you need to go in every 2 months or so just to get a tiny bit of ends cut off.
I think that's BS. When you're trying to grow it out, don't cut it. Ivarei is right about not washing it every day, that helps it stay heathy better than any shampoo, and your ends do not get so bad so fast. If you get it trimmed every 2 months, you will find out that in a year you hair hasn't grown at all, because you're getting all your growth cut off all the time.