How long drive time with kids without wearing out?

Probably too late now, but don't you think trying out a 2 day trip first, then a month later going for a week, and gauging the children how they are 6 hrs in, then 10 hrs in, etc, it should be apparent when YOUR children hit a wall and can no longer stand riding in the motorhome.

I could say my kids are good for 15 hours, but your kids might not.