Suppose you want to get a chapter or article written in the next two or three months--that's the situation I'm in. You can think about that in terms of hours at the computer it will take you to write it, or in terms of months. It might only take your 30 hours to complete it, so two months seems like an absurdly long time. 30 hours in 60 days! What could be easier? That's an average of half an hour per day. Yet most scholars take much longer to produce their work. Part of the problem is that there is too much time. In other words, it is hard to figure out where those 30 hours are going to come from, simply because they might come anywhere. 12 a.m. to 1 a.m. on Wed. Sat. and Sun., every other week, and 1-5 p.m. every Sat? That might get it done, as long as you realize the weekend you lose to house guests... The key is finitude.
That's why I've been keeping track of my work during each week. Months are too long for this kind of planning, but if every week you make significant progress, then there shouldn't be a chapter that takes more than a month or two.