My last set of cameras for shooting weddings was a pair of 5D3s and Canon batteries. I shot for the entire day, raw+tiny jpeg with some chimping when changing conditions, but never with live-view, on a single battery and never did it get below a quarter charge, usually between a quarter and half. I did of course always have a second battery for each camera but only in case of failure. They never got used but of course I rotated them for the next shoot. As well, I almost entirely shot the day with IS lenses which of course take their toll. Personally, I would be quite comfortable spending a day somewhere with a pair of batteries. Enjoy your day wherever it is you're going!