I sew a lot. For my Viking which I got 37 years ago I would go by the sound and "feel" of the sewing. It told me when it needed a tune up, much like a needle will tell you when it's dull if you forget to change it. I could usually go around 18 months between tune ups when I was sewing hours and hours each day. I cleaned out the lint and such along the way. If a person didn't sew as much as I do then it would go longer. For my Featherweights I do the maintenance on those with regular oiling and cleaning. I just recently got a Janome 8900 and at this point haven't got a feel for when it will need to be serviced.