Why stop at novels, I wondered?

It doesn't matter what it is you write--biography, self-help, annual report, historical non-fiction...it comes down to the same thing that also occurs in other very different lines of endeavour: if you've got three months, that's likely what it'll take to do the job.  If you've got a year...the writer will take a year--unless there's another compelling idea/concept/contract that intervenes. And then it's "get this done quick" to get to the "other stuff." 

It's a human condition.  The external "do-this-or-else" deadline is a great motivator.  Self-set deadlines, though...hmm, not so much.