It should take years to MASTER.

But it should take less than a few months before you become (very) effective with it (particularly against someone who is untrained). (Think OJ Simpson for a second)

But the base problem with a knife is that it is (*) a melee weapon rather than a range weapon...you have to be close to your opponent.

Anyone with a range weapon had the possibility to kill you before you close with them.

(*) Throwing knives aside.