Why not have it last for about 8 posts or something and give it an 8 post cool down? that way it lasts some time but eventually it's effect wears off and it takes its toll on him for awhile before he can use it again...*shrugs* you probably want it to last for a bit of time (obviously not to long or to short) and you likely don't want the effects to last to long either so basically settling in the middle with this one xD