I have a system for cleaning my machine and changing my needle. I have my bobbins in a case and only have 5 bobbins wound at a time. When the 5th is used I clean my machine and then get 5 more wound after it is cleaned.
At this point I put my needles in the tray beside the 5 bobbins, which lets me know that once those 5 are used I not only clean my machine, but change my needle.
So, clean after 5 bobbins and change needle after 10. You don't think it makes a difference, but it REALLY does (sort of like when you change your rotary cutter blade and are in awe at how easily it cuts through fabric after the change).
Suzanne