War and Peace wasn't written in a day. In fact, it took Tolstoy six years to write the novel. I'm not sure if anyone knows how many hours he dedicated to writing each day, but I assure you, he prioritized his writing time.
If you have a busy life and have been thinking of writing a book, you're going to need to do the same thing as Tolstoy. It'll be easiest if you embrace this concept: in order to create something of value, you're going to have to give something up. There's no way around that.