How often to demagnetize mc cartridges?

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.

I use the Cardas Sweep Record every few months and find that it is
worthwhile to do. But my limited understanding is that this is a very
different (and probably more lightweight) process than the demagnetization provided by the Sumiko Fluxbuster and that kind of direct demagnetization, particularly if overdone, can be harmful. 

AJ Van den Hul, for example, strongly recommends against 
demagnetization with stuff like the fluxbuster. 

There are some interesting comments below from JCarr of 
Lyra from a post at Audioasylum. 

https://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=61192&highlight=degaussing+demagnetizing&r=&session=http://

I was told by Dynavector that their cartridges do not need to be demagnetized. I have a ZYX that I have never demagnetized - sounds fine to me. 

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 :)