I clean my machines after finishing 2-3 bobbins. Depends on the machine, my BabyLock could be done after each bobbin.
I have had a few ladies recently who "clean" their machines and then ask me why it is not working correctly. Both have drop in bobbins and did know to take off the face plate and lift out the bobbin holder. The lint was so bad it had pushed the bobbin UP. A good cleaning and change of needle and all was well. As a thank you, one of the ladies gave me an OTT light, awesome, you can adjust the height of it.