When I started riding about 12 years ago and was overweight I used to sit 90-95% of the time on hills. Now when I am in a much better shape and 50 lbs less I stand about 50-60% of the times on any climbs, short or long.

Right after training doing intervals or running (improving anaerobic capacity) I find it is easier to stand and pedal using more lungs then legs. In the beginning of the season when my anaerobic ability decreased I find myself sitting more on hills and grinding, standing up only for short durations to change the body position, use different muscles and cadence (my lungs could not cope with longer duration standing and I get tired fast).

The better my lungs are trained the longer I stay standing climbing.