Can You Tell How Long...?

Can you tell how long a book is going to be while you write it? Can you tell how long a life is going to be while you live it?

No.

I just found out that Jeffrey Deaver (The Bones Collector, etc) lives here half the year in Pacific Grove (other half in NYC). He writes an average of one book (best-seller, invariably) a year. Good, thickish books that are sizeable enough to be used as doorstops or bludgeon-type weapons. How on earth does he do it?

Louis L'Amour usually wrote his books in about 3 months apiece. Issues of templates aside, how did he do that?? A book in 3 months is a tremendous rate.

I need to finish this book I'm writing, or it shall finish me.