I like to begin writing by working on things that are already written, fixing sentences. Then I move on to writing new but complete sentences out of rougher notes. Finally, at the end of the writing session I brainstorm, writing down things that occur to me, new ideas, without worrying too much about where they might fit.
My logic is that the "warm-up" period is not very strenuous; I can tweak sentences that I wrote before without too much exertion. Writing new sentences is a bit harder. Once I'm warmed up I can do that. Toward the end of the session I don't have much to lose. I've already written enough for the day, so I don't have to worry about anything else.
This three part structure might vary. For example, a paper I have been working longer on will need fewer new ideas and more tweaking of sentences already written.
(Notice that this is a reverse order. You would think the logical order would be brainstorm, write, revise. What this means is that the same material I brainstorm today will be written later and then revised yet another day.)
Typically, I write between 1 and 3 hours. Hardly ever less than 1 or more than 3. 1 and a half is good; 2 and a quarter also.