It depends on how healthy I've been. Whenever I get sick for a week or more, I'll notice a jump in split ends a month later. If I haven't been sick, usually my hair stays pretty healthy, and even after a year, if I look at the ends of my hair, I really won't see split ends. I don't think there's really a reason to cut it if you don't have split ends, unless you just want to maintain a certain length.
When that's the case, I don't tend to trim my hair at all (I'll go a year or three without cutting), and my hair goes from chin length to waist length before I cut it again. It works out for me though because I like both shorter and longer hair styles on me, and I don't like to go to the hair dressers that often. (Right now I probably have 10-12 inches or so I can cut off) I think it's been almost 2 years since I last cut it.
edit: actually I just looked it up, and average is around 6 inches/year hair growth, so yeah that's probably about right then.