I took mine in after nine months and got it back the next day. (I arranged ahead of time to be able to do this - kinda like I got my baby an drs. appointment.) When I picked it up he showed me the lint and stuff they got out of areas that I could not reach in normal maintenance. He said that perhaps I should take it in every six months instead of nine. But, I use my baby just about every day and had already sewn and FMQ seven quilts on it before it went in.
When you quilt your quilt, either stitch in a ditch, FMQ or just plain straight lines, your needle is pulling the lint in from the quilt batting and that can build up inside your machine. That makes maintenance a more frequent necessity than if you were just sewing garments.