I do 20-24 hours a week as a freelancer/consultant. And it is definitely productive experience. I have at times hopped to 40+ hours and never enjoyed it. I tried 30 per week but that was not better either. These 20-24 hours are highly productive and I spend the rest of the time with family/friends - or reading/experimenting.

The downside though - I sometimes feel I am wasting my time (because not many do so) and occasionally worry about opportunity costs.