Mine was 19 hours, driving a van full of kids to a robotics competition and pulling a trailer full of gear. (Nobody else wanted to pull the trailer)
I also did back to back 18 hour drives once.
I can't handle back-to-back long days on the road. My cross-country driving secret is to sandwich two 18-hour days around one shorter day (8-12 hours), then use Day 4 (if necessary) to finish the last leg of the trip.