A question for anyone who has written a novel length story... at what point did it hit you that you were just done?

I'm a teenager, and I'm entirely new to writing. I am trying to write an original novel, and I have written a couple of short stories in the past. But I know that the story I am working on right now is my best so far. I have a pretty good idea of how I want it to start and end... but it's the middle that is the problem. I don't know when to quit. If I try to write too long, I will burn out.. but if I quit writing too soon, I will miss a lot of the story. I know there are books on how to write, but I thought I would ask here as well. Thank you. (Also, if someone could give me a good link to a how to write book, I would be much obliged.)<br><br>Edit: This post has been locked due to a lack of context, but anything written in the comments is very much appreciated! Thanks all!