I've talked a lot about book length, but that's because it's such a fine science. You always want to leave your readers wanting a little bit more. They should feel just a little sad, and not at all relieved, when they get to that last page. That's why all authors have to make sure their novels can pass the boredom test. There's only way to administer the test: you have to read your own book until it almost drives you out of your mind.
Back Off, I'm Editing
Editing a book basically means reading it -- over and over and over and over again. In fact, four times isn't really enough to give your book a thorough edit. Seriously, it's not. Because even if you can somehow get rid of all your mistakes, proofread all your grammar and punctuation, check or plot holes and eliminate all the excess stuff you don't need, you still have to make sure your book can pass the boredom test.
And in order to conduct that test properly, you're going to need to read your book a good 5 times or more. Because if you can read your own book at least 5 times without stopping in the middle, hurling an ereader against the wall or falling asleep, then it has passed the boredom test. Basically it works like this: if you get bored reading your book, everyone will. Most people read their favorite books 5 times or more. If your book has any chance of ever becoming someone's favorite book, then the boredom test is necessary.
In fact, it's an integral part of editing that book. The whole point of editing is making the novel as perfect as possible, and that includes capturing and maintaining interest. So go read your book until you think your eyes are going to bleed. As long as you don't feel like sticking forks in your eyes as a result, your book is on the right track.