It’s simple really.  Assuming you’re a solid employee and clearly not an axe murderer you should be 50% ready for the promotion and 50% not ready.

If you’re 100% ready you’ll become bored because there won’t be a challenge.  I really believe that the best time to promote someone is when they’re 50% of the way there.  This way, they have room to grow into the position and learn on the job.  As a manager you’ll still be able to provide value because you’re going to help them fill the other 50%.  Think about it.