@Doc,
How are you inflicting it? If it's via a MagicEffect with 'Recover', it will work just like a normal Health Damage effect--once the event is over, the damage will be done and it will be instantly restored to it's usual value. Are you sure it won't persist past a save and reload? I've never tested, but it seems like it should, since it's just an attribute like Health itself...
If you actually want it to progressively raise back to normal, though, you'll definitely have to script it (or keep re-casting slightly less effective versions of the damage effect).