I don't think there are any set hours. When my husband and I got married we had an open bar for about 7 hours. As long as there were people at the reception, they were drinking. My brother got married in Laguna Beach at a hotel, and we were drinking before the wedding and for about 4 or 5 hours at the reception. I believe if you have it hosted by someone serving the alcohol, then of course you are going to pay by the hour, and you may want to limit the amount of hours to avoid such a huge bill.