I suggest Genjutsu may not last forever, and must have a maximum post duration. Logically, in Naruto if Genjutsu lasted forever, then one could Gen-rape the afflicted for eternity. If there is a Duration, then the Genjutsu must have a maintaining fee in chakra. This could simply be a chakra per turn cost, of course Genjutsu will have to be stating how long actively they're present.