While we don't have a formal policy, you have to comply with the wage and hour laws of your state.  Legal/Risk Management here said that no volunteer can serve as many hours as a regularly scheduled paid employee during a pay period---a long way of legalize to say less than 20 hours a week.

Reason being, someone might sue saying that their position should be paid since it is regularly scheduled and of equal number of hours.

Our unwritten rule is a max of 18 hours, although we have never had anyone do more than 12 in a week, and those are special projects done in a month to meet our 48 hour minimum requirement and at the request of a department.

Frank