I like to get my hair trimmed at least every 4 months or so. You can go longer if you don't have any split ends. But I like to prevent split ends before they happen.
Contrary to popular belief, getting regular trims does not make your hair grow faster. It does, however, keep you from getting split ends, which as they worsen, break the hair up the shaft. So regular trims keeps your hair from breaking, therefore giving it the illusion of growing faster.