Sure! How about for every day the Leviathan has possessed the person, it becomes exponentially difficult for Zelos to exorcise them. Maybe it drains him more, maybe it becomes more risky and there is a greater chance of harm to the victim? I'll leave that up between you and the player of the possessed character. If someone has been possessed for a day or two, it's pretty easy. After that, maybe Zelos has a harder time recovering from the strain and needs to take a break before attempting again -- just to avoid a sort of assembly line of possessed to Zelos for treatment XD