It depends on your hair. I have done two experiments of going 1 year without trimming, and both times I had to cut more than the year's growth from the ends, they were in such bad shape. Not only do my hairs split, but the split ends split, and then the ends of the split splits split and ... You see where I'm going with this.
I do much better if I get a small trim about every 3 months. It keeps my ends in good condition so that the damage doesn't spread and cause my ends to break off and go split-crazy.
I know of other people who haven't had a trim in years, however, and their hair is extra-special long and lovely.