Keeping your hair trimmed will promote healthy growth while eliminating split ends.  How often you should get your hair cut depends on length, but as a general rule:

SHORT HAIR:
Very short styles require constant upkeep, so you are most likely going to want to get a trim every 2-3 weeks to maintain a health looking style.  Remember, if you are trying to grow your hair out, you still need to get it trimmed to promote healthy ends.

MEDIUM OR LAYERED HAIR:
Layered hair also requires some significant maintenance, needing a trim every 4-5 weeks to keep the layers intact and ensure for healthy volume!

LONG HAIR:
If you keep a long style and your hair is healthy, you can get away with a trip to the salon every 6-8 weeks, that is, unless you have some color to maintain.  TIP: If you see a split end, cut the individual hair right above the split with a sharp scissor to keep your style looking fresh!