How often to demagnetize mc cartridges?

Nandric and I agree. As to the cardas  test record, I have always felt that the benefits derived from using it are as much due to exercising the cartridge suspension over a wide range of frequencies as they are to any demagnetizing effect.

To the OP: I am surprised that your reading of these responses would lead you to the conclusion that you should use the fluxbuster every few weeks. My reading leads me to conclude that it should not be done frequently and then only for MC cartridges with an iron core coil. I would define “not frequently“ as every six months to a year, at most.

I don’t seriously disagree with those who say never to use a flux buster like the Sumiko. As I mentioned, I only and occasionally use the Cardas test LP for that purpose, and it does seem to rejuvenate MC cartridges. My rationale is that I am fairly sure that playing an LP is not going to damage the magnet in the cartridge. It is probably a very gentle process, if it actually does any good at all. But one cannot say often enough, never demagnetize an MM cartridge. The same probably holds for an MI type, although maybe less dangerous in that case. In fact, I once owned an Aesthetix Janus preamplifier which has a demagnetization circuit that you can switch in. I was so leery of accidentally damaging any of my precious vintage MM cartridges that I disabled that circuit.

I don't know how the cartridges on your extensive list are made.  Except for the ART7, which I do know has an amorphous core coil. I think that one contains no iron, no magnetic materials, does not need fluxbusting ever.  MM cartridges, of course, should NEVER be demagnetized, as you may kill the entire beast very quickly.  I have the sameSS-modified Grace, based on a Ruby original.  It is excellent. As long as you don't demag an MM, I don't think you can do harm to an MC by fluxbusting. I use the Cardas test LP once in a while for that purpose.

Depends on how the coils are constructed. If iron core, the core can be magnetized. Then an occasional hit with flux buster is worth doing. But not very often.