I think how often you get your hair trimmed and need to get your hair trimmed depends on the condition and health of your hair. If you have very damaged hair with a lot of split ends then until your hair improves you should get the ends trimmed every time they start to get ragged and split. You can usually tell if you are in need of a trim by how your hair feels and looks.

For myself since my hair is in good shape I only go to a salon to get my hair trimmed twice per year. Between my trims I use my professional sheers and once in a while I will go through my hair and cut off any split ends if I see them. This helps keep your hair and ends looking great and you do not lose much length. If my hair looks and feels like it needs a trim more often than this I will go in for one but usually for me twice per year is enough. You have to be careful where you get your hair trimmed because some hair dressers tend to go crazy cutting off more than need be.