How often to demagnetize mc cartridges?

Never is the correct answer. Worse than stupid concept perpetrated by people who have no idea what they are talking about. If you have a reel to reel you can demagnetize your tape heads. If you don't, throw that thing in the garbage. Your going to hurt yourself.

NEVER demagnetize anything which already has a magnet inside it for its normal operation.  Otherwise you will destroy/weaken the magnet.  This includes moving coil, moving magnet cartridges and speakers as well as your credit cards :)

Fluxbuster works fine on HOMCs.
Use it whenever you think it will improve the SQ. No set answer, every system and ears are unique.
Did you get the manual?

Depends on one's amount of playback. For me, it's every 6 months.

Demagnetise MCs?!? 🤔 
Ah... never! Ever. 😱 😲
M. 😌