Barry Eisler writes:
It took eight years from writing the first words to the first sale of rights of the manuscript that became Rain Fall, featuring the half-American, half-Japanese assassin John Rain. During that time I was busy drafting and negotiating technology licensing agreements with law firms in Silicon Valley and Japan and my daughter was born along the way, too, so I couldn't write as steadily as I would have liked. I revised the manuscript so many times that effectively I wrote it anew several times, and learned a lot about the craft of fiction in the process.
My subsequent books have all taken a year each. I think the acceleration results from: (i) I know what I'm doing better now; (ii) I'm writing full time now; and (iii) I write a series, so I already know the protagonist pretty well and don't have to figure him out from scratch. When you're writing full time, a year is a lot, I've found (if it weren't for all the promotion, that is).
Barry Eisler is the author of the John Rain series, which has been translated into over a dozen languages and optioned for film. The latest book in the series, Killing Rain, will be published in June.