How often do you demagnitize your MC cartridge?

I've never done it.

Copper is not a magnetizable material. It cannot become magnetized. The coils in a cartridge are made of copper, usually very high purity. The magnetic action that acts on the copper coils is an inductive process that does not result in any "permanent" magnetization of the copper. It simply induces a current in the copper coils in an electromagnetic process like a generator. Copper is not a material that can become magnetized. Copper transformer windings don't become magnetized, copper speaker voice coils don't become magnetized, copper commutators in generators and alternators don't become magnetized, and copper coils in cartridges don't become magnetized.

Why would there be a need to de-magnetize something that is not magnetized in the first place? Now, maybe there is something else around the cartridge, like the screws or other hardware that can become magnetized, but not the coils.

Marakanetz, yes of course there are magnets present in the cartridges. But they do not cause any permanent magnetization of the coils. They cause the coils to be energized by the movement in the field, and induce a current. But, when movement stops, the signal generation stops, and no magnetizing force is left residing in the coils, other than that which is created by the magnet's fields.

However, there is some possibility that there is some content of nickel or other magnetizable metal in some of the solder joints. That could become magnetized. However, I really think that is a low possibility, and would only comprise a negligable amount of material that could be magnetized extremely weakly, and is not in the vicinity of the main field anyway. I think that is not a real factor in this, but I mentioned it anyway.

I'd say six nines is pretty pure copper.