Depends on the club. My club charges three hours for a full day at weekends (in theory that is - last time I took an aircraft for a full day I only flew 2:45 and they didn't actually charge me for the extra fifteen minutes), but no minimum during the week when there are likely to be aircraft idle anyway.
It's basically "how much money do we lose by letting this guy take the aircraft away", and trying to make sure that the answer is zero.