Our philosophy is that we want to do some interesting volunteer work that will be rewarding to us. We look for opportunities that meet that criteria. I guess that we have some "maximum" hours, but mostly we want a nice site, and the "work" is a separate consideration. We have had places where we worked a little as 20 hours a week total that we liked less than our most recent adventure where we worked a total of 48 hours a week. More important than the hours is the job, how much we enjoy what we are doing, how well we like the management and other volunteers and paid staff.
Full disclosure: We don't full time. We travel about 6 months a year, and spend 3-4 of those months in volunteer sites. So our considerations may be totally different from a full timer.