Well I'd guess I used to throw about 25'. Another factor would be the knife though. A lot of the "throwing" knives you see are so light that they'll lose too much energy to be usefull at a long range. No science there, just opinion.
The Knife I used at that range was about 16 oz. and would go through the 3/4" plywood I used as a target. The handle would stop it from going all the way through. We called it "the armor piercer"
I could hit a 12" circle pretty reliably, so that seems accurate to me.