With mine ( Viking a purchased in 2004) I take it in when I need to- - sounding a little rough, something not quite right—
things vary year to year. Some years I put a lot of miles on my machine and I take it in for a tune up, good cleaning, maintenance check up, other years life is busy in a different way & I don’t sew that much- I may go a couple years or more without taking it in. I always clean everything on it I can regularly and take care of it- I would like it to last my lifetime so I try to take good care of its needs.