Hmm, okay, that's an interesting point. In deciding upon how much time to give, I approach my volunteer projects just the same as I would any other task; putting in as much time as it takes to get the job done to the best of my ability.

I would say how much time most people give away is totally dependent on their other responsibilities at the time and how much of their personality is geared to giving. If you're a giving person by nature, it's easy to give until you meet a point of diminishing returns.