My computerized Pfaffs get cleaned/serviced in AZ once a year for $85. They always call if parts are needed - but none has been so far. Time depends on how busy they are - never more than a week. They have a blackboard in the shop that says 'Machines brought in today will be ready XXX', so you know before you drop it off. There's a LOT of dust in the air in AZ, so cleaning is a smart thing to have done professionally.
I do the same in CT - but have a new dealer, so I don't know the charge yet. I have 2 machines in CT, so I'm never without a machine here, knock on wood!
I use my machines almost daily, and I clean the lint out every time I change bobbins. I think the annual cleaning/servicing is worth it to stave off problems BEFORE they happen. I really trust both of my dealers. They're in business for REPEAT business.