It depends ...
I used to bring my Husqvarna in once a year like clockwork. After the 3nd time my dealer asked me "why"? The first time he understood - the first "tune up" was free with the purchase. The second time he just did it. When I came back the third time he flat out told me it didn't need it after he opened it up. Not only did I not use it enough during the course of a year, coupled with the fact that I am a FREAK about keeping my machine cleaned, and he said the Husky's don't need maintenance as frequently as other machines.
I've also got a Janome 6600 which I use more often than the Husky, but I'm still freakish about cleaning it. Had it about a year now and I'll bring it in when I'm done with my current project for a once over and a blessing. I'll ask them at that time if they thought it needed it and if not, I may stretch it to 18 or 24 months until it's next visit (given the same amount of usage).
I know I don't put in as many hours on my machines as some of the ladies here on the board. I go days, sometimes even weeks, without even turning my machine on. And as I said ... I'm a freak about cleaning the suckers.
So ... it depends :)