Of course they said I should have it serviced every year, but for the price of having it cleaned yearly I buy a second machine after about six years. I don't want it to end up in a landfill because I didn't take care of it, but I don't think it really needs it yearly. Every three or four years seems to be enough, and from the other posts so far, maybe even that is more than needed.
If a machine is used a lot or not used much, it probably needs more frequent servicing than one that is used a little regularly..