If you're like me, you have at least six book ideas buzzing inside your head at any given moment. I read once that James Michener did all the research and outlining for IBERIA years before he ever got around to writing the book, and I know how that goes. It seems like there's never time to develop that clever idea that I wrote three chapters of one week, or start on the one that comes to mind when I'm driving long distances alone, or rework the one that isn't bad but could be improved now that I'm farther down the "write" path.
In my mind, I picture taking a sabbatical from life and just writing for a month, or six months, or in true Hawthorne style, a decade. The line from "The Walrus and the Carpenter" comes to mind: "If seven maids with seven mops..." Could I really get on paper everything I want to, or would more ideas just keep cropping up?
I think the latter, which is why I have a sort of unwritten rule that I finish one project before I start another. The rule is sometimes temporarily broken when an idea is forceful enough to take up my whole head. In that case I may give myself a day to get the nuts and bolts of it recorded or write a couple of chapters to see if what's in my head makes sense. But then I make myself go back to finishing whatever is Job One at the moment. Otherwise, I could have the first three chapters of a hundred books and the final chapter of none.