For me, it takes about a year to write a paper —- if I am lucky. And that is just to first submission to a journal. After that, god knows how long it will take to make in into print. Sure, I can write a first draft in a few weeks. But that first draft will have lots of holes, be poorly written, and be full of problems. It will take many seminar presentations, conferences, and rewrites before it is a polished piece.
So I am a bit nervous about starting out a new paper. My papers rarely end up the way that I planned them. Once I get going, I find out what the paper is really about. Usually the paper starts short, grows, and then shortens back up as I finally figure out the real point.
My rule is that I do not start a new paper until an old one is accepted, or close to being accepted. Like many people, I keep an ideas file although it is not really necessary. Ideas are easy. Papers are hard.