As high as you can.
Zenith is the opposite of Nadir.
Figuratively, the zenith is how about 1500 bloggers are feeling right now. They've posted their final Z post and won the challenge for 2015.
In books, the zenith is the highest emotional point of the story. It's the point where everything is going right. If that point is in the beginning of the book, then be ready for a crash, or at least a collision. Something is bound to go wrong soon.
If the zenith is in the middle of the book, it's usually in conjunction with an anti-climax.
Let me detour a little : A climax or turning point normally has a crisis attached. Something is going wrong and the main character needs to take action to solve, fix, escape, or otherwise deal with the crisis. But if the main character is at an emotional zenith, then there isn't any visible crisis. Therefore the turning point is generally an anti-climax. After all, from a zenith, the only way is to go down.
OK, we're back. Did you enjoy that detour? Good. Moving on.
Often a main character at the zenith in the middle is feeling overconfident. "What could go wrong? My plan is perfect." That's when something is about to go wrong.
When the character is at a zenith at the end of a story, it usually means that a nadir was in the middle, or at the beginning of the story. I wrote about the Nadir, or "darkest moment" in my N post.
I guess what we see here is the need for balance. Also change. Think about it: if the character is at a Zenith all the way through the story, then you really have no story. Or if you do, then it's awfully flat.
What's your zenith?
I write about, with, for, and around kids all day. (Well, maybe I do the dishes too. Sometimes.)