This is such an individual thing that no one answer is good. I've done as little as 100 miles or as much as 850 miles. The trick is to determine your planned stops first - parks, entertainment, or other. Then pick out comfortable driving distances in between them. If I'm just driving for distance I may go 300 to 500 miles in a day. Then for planning purposes I'll locate a park less than 400 miles, one near 400 miles and one beyond 400 miles. That way I'll have some adjustment built in depending on how my day's travel has been going.
