How often do you demagnitize your MC cartridge?

In most cases it's not recommended. The cartridge normally must be magnetized. Even if you believe on such drastical improvements tend to do it not any often than twice per year no matter how often you play your records.
Don't ever do it with MM cartridge and keep it away from strong magnetic fields.

I've never done it.

Don"t ever do it!!!!!

It should only be done immediately after returning from alien abduction.

Do not do it. Cross-ground them and play them for 30 sec. It WILL accomplishe all clearly auditable benefits and you will be all set to go for another couple weeks.

Rgs,
Romy the Cat