I make 3 at a time when I'm piecing a top. When I do fmq for a queen size I wind 8. When they run out, I just pop a new one, but I take a few seconds and swipe the bobbin area for dust. Usually after the 3, I do a full cleaning (take off the plate and dust everything and oil).
My Juki likes to be clean and oiled per the every 8 hours on the manual. And I'm not going to push it, I want it to last forever.