How long drive time with kids without wearing out?

Our trip last year was almost as ambitious as yours. We had two 9 year old boys with us in a crewcab. We rode hard and long for 3 days in the beginning and 3 at the end. Not one peep of complaint the entire trip. Personally, I didn't once feel tired except for the long last day.

To each their own. I know my kids are troopers.