How long does it take to write a book?

I gave myself four months to write my new J. S. Haldane biography Suffer and Survive, though a deadline extension was always likely and I grabbed it. I've just taken a week over a new draft following my editor's edits - submitted just moments ago. Words from editor and agent such as 'fantastic' and 'splendid' gave encouragement to that final spurt.
So the writing took six extraordinarily zealous months, following two years of research. Six months seems as good as I can do with a book in terms of speed.
One curious feature this time around is that I have lost my sense of time passing. My internal clock keeps assuring me that it is still October 2006. I've found myself asking people not only the date, but the year. Maybe that's just a result of living so exclusively in somebody else's era all this time, while working to bring them alive into my own.
It's been worth it. J. S. Haldane is a veritable hero.

The picture is of J. S. Haldane at the temple of Sounion in Greece, taken on his last dramatic world tour shortly before he died in 1936