Needed 6 hours, though many people take longer, as I discovered. The schedule was supposed to include the Narrows, but a rainy forecast nixed that option. Instead we set up 3 shuttle cars at the Grotto picnic grounds. and returned to Lava Point to start the hike. Half of the hikers had already started on the hike about an hour before the shuttle drivers got back to the trail head. It was now around noon.
Large groups are slow to get started and hard to coordinate.
For this reason, Zion now limits backcountry groups to 10 or fewer people. We had over twenty in our group, which was OK then. So the rest of us start off into the gray afternoon. We catch up to, then pass the earlier group as they rest and eat. Two of the shuttle drivers hang back with them.