Good point. As anyone who has written an essay before can attest, if you have it all in your head before you start writing it can flow out from your fingers remarkably quickly. If you're still piecing it together as you go along it will typically take an order of magnitude longer.
Some clues are available in this video: https://www.youtube.com/watch?v=4XpnKHJAok8 (Tech Talk: Linus Torvalds on git). There's nothing conclusive about the length of the pondering, but he does say the writing itself took two weeks:
"I decided I can write something better than anything out there in two weeks. And I was right." [1]
Between he publicly deciding to abandon BitKeeper and making git there were a few months. I imagine he would start pondering it right away, even if only to get a feature list to look in a replacement.