For MM cartridge if you de-magnetize the cartridge simply will not work and the magnet isn't going to move.
For MC cartridge you need demagnetizing no often than once per 400 hours or OK not to demagnetize at all since demagnetizing brings almost no difference to the playback.
For MC cartridge you need demagnetizing no often than once per 400 hours or OK not to demagnetize at all since demagnetizing brings almost no difference to the playback.