[Shujin]

Since its an Invocation I'd probably am sooner going to say 20 than 15, since I do not really think the regen is that good, and else it would pretty much only be a lightning buff stick,

Another idea I would have is giving it the invisible blade buff treatment from Spellthief, aka, having the blind fighting trait half the effect.

Cause I agree in PVP a 25 hit debuff is pretty nasty, I think its fine in PvE though, thats my main reasoning for saying that.