Both the dealers I have bought machines from say about once a year, but it does depend a lot on how much use it gets. My newest machine I've had for about 10 months and I'm thinking of taking it in very soon because I've been using it heavily all spring and summer.
The last time I took a machine in I had just kept running it and running it because I didn't want to be without it and frankly didn't want to cough up the bucks to pay for it if it wasn't really necessary. It finally started acting just a bit sluggish so I took it for a cleaning and tune-up. When the guy asked if it had been a while, I realized it had been over 5 years! I clean the lint out of my machines religiously. It just gets in places I can't reach. It was amazing how much better it sounded after that. I promised my new machine when I brought her home that I would never make her wait that long between servicing!