I clean mine once a week, whether it needs it or not.
Easy way to clean out between everything in there if you don't want to use your vacuum. (I won't use mine because I'm afraid it might suck up something it's not supposed to)
Anyway...if you get a Q-tip, put a little dab of oil on it and GENTLY put it down inside the bobbin area, give it a little twist, it'll grab the dust in there.
I also won't use canned air because sometimes "wetness" will come out. The man that I take/took my machines to for cleaning says to NOT use canned air for that same reason.....