If it is in an area I really want to be, more hours are no problem. 40 hours in yellowstone or yosemite? Definitely. But then, chances are, I would not take a job in a place I really didn't want to be.
I volunteer because I love the place, the park, the people, etc., not because I expect to be paid or make money. I don't look at the work/volunteering as payment for the site; the site is a perk for volunteering. I guess it is based on your priorities. If you are doing it just to have a place to live, doing less work makes sense. If you are doing it for the experience, the amount of work doesn't make a difference.