I'm keeping with the Throwing Knives already in the game. 3-4 of the Poison Throwing Knives almost always poison the target (if it's a wyvern, 1-2 if a normal enemy/drome), 3-4 Sleep knives put it to sleep, and so on.
And with this particular item, unlike the other knives, the more times hit with this knife (after the affect has taken affect), the stronger the effect is. So, 3 lowers it's damage by a bit, 6 lowers it even further, and after 10 or so, it will do around half normal damage. I meant to add that up there.