Actual time I play the game varies. For me to take good notes I need to be interested in the game. And for that to be the case, I need to be flexible. That said USUALLY not less than 3 hours.
I'm normally about 100 years ahead in gameplay of what I'm writing about. I use chapter divisions based on game events. I try to keep the chapters more or less similar in length, but I don't have a word count going on them and I don't mind if one covering a long period of peace is longer than others. Generally similar length chapters go over better than totally irregular lengths--though an occasional "interlude" marked as such can help move things when you have less than a full chapter's material.
Pacing the AAR is entirely based on events. I could fly through time sometimes in Nippon due to a large variety of similar events (that is, revolts). This AAR I'm constrained to do things (at this point anyway) a little more slowly because so much is happening in so short a period of time. To me, flexibility is important here. I don't like AAR's that compress 50 yeas of war into a paragraph. But if nothing's happening, don't feel forced to manufacture events to make something happen (though sometimes a couple zany events in an otherwise quiet time can be used for something special, like my "Mad councillor" in Brandenburg).