For weddings a minimum of 4 hrs; Ceremony 1 hr (+ up to 1 hr for rehearsal free).
My theory is not to book for more hours than one needs, but when nearing the end of the last hour (4th), then consider O.T. by the hour. It works for me. And, since I am getting up in years I'd just as soon have the option to continue beyond the normal 4 hours.
Birthday and Retirement Parties; still want the 4-hr period but will settle for 3-hr at the same rate (which is lower than the Wedding Reception rate). Again, Overtime is an hour-by-hour decision.
I did an 80th Birthday party in early May that was scheduled for 5.5 hours but to my surprise they opted for a 1-hr O.T. extension. It was an outdoor party under a large tent and the weather wasn't bad at all. However, that extra hour nearly killed me...I didn't want to do it but when the guy comes up and flashes $100 bill in your face and says, how much for an hour of overtime...and would this do? Hard to say no. The birthday boy (80 yr) was still partying to the end. Before accepting the O.T. I polled the Client because it was a friend of the family who wanted to extend the party. There comment was..."if you've got the money from him, it's up to you." Needless-to-say, I played.