The time spent on HubPages for me is around 25% I have been tracking results with Adsense for the last few months. So far I have earned close to $40 with my my few hubs. Am also tracking a few other web sites from which I earned about $4.00 combined in that same period of time.
From that data, it looks like I need to be spending a much higher percentage of my time on HubPages. With their high page ranking and almost guaranteed traffic, from an earning sense it sure seems the way to go.