30 min it's plenty. More important is to do it regularly. If you put the bar too high and overdo it, you'll feel constantly tired and you more likely to loose motivation. Exercise, apart from fitness and health benefits should also be a relaxing and pleasurable time for you.
I'll repeat what's been said, 30 minutes is plenty for a workout especially if you really push yourself. I personally only exceed 30 min if I'm walking.