I've gotten pretty good at being able to tell when I have 20 minutes left--which for me is ideally the cutoff for bringing up a new topic (though of course I don't always follow my own rule). I tend to keep a closer eye on the time in the last stretch, too, as I don't want him to have to say "We need to stop."

I'm worse at judging the earlier part of session. Sometimes, I feel like I've been rambling/we've been chatting a long time, but then I look at the clock to see it's only been about 10 minutes. Other times, it's more like 25.