I usually try to go by the calories burned to an extent. I don't believe it's completely accurate, not sure how accurate it is? I don't think I've done a cardio session beyond 35 minutes. I know Luna just put in 50 minutes!
I went for a 600 calories goal, which is what i normally do. Also put in a bit more time yesterday because it was the first time i did the eliptical and i really enjoyed it. Sometimes cardio is just easier than others and thats when i spend a little more time on it .