How often to demagnetize mc cartridges?

Never done it.  No need. 

Did get the manual but it doesn't mention or recommend time between treatments.  

Thanks for your responses everyone!  I think I'll just try using the fluxbuster every few weeks or so.

Is the primary question not about ''coil core''? Iron core is used
in order to increase the output.'' Coreless'' kinds as the expression
suggest have no ''core''. It is this ''iron core'' which get ''magnetic
saturated'' and may need demagnetisation? 

When I responded earlier, it was because of my use of the Cardas lp.

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.