I heard elsewhere that Medusa's stone does 1/3 of a giant's (idk if any upgrades) health, and 2 marrowkai can control giant basically forever, with enough micromanagement.
Against an upgraded giant it does no where near that, maybe vs un-upgraded, but un-upgraded are not the concern :)
Yes, I think 2 marrow can control 1 giant indefinatly - its the other 5,6 or 7 giants that are pounding on your meat shield the issue :)