Experience. And double whatever number feels right, and possibly again. I try to break down such a task, often a large one, into parts no bigger than about ha

Experience. And double whatever number feels right, and possibly again. I try to break down such a task, often a large one, into parts no bigger than about half a day, working out the overall structure and some of the detail of each part. Then tot up the time for those parts and double it (or more) as above.