I found on my trip from St. Petersburg Florida to Vancouver Washington last year if I used 50 miles an hour as an average, six hours or 300 miles a day.Was very doable I came across to the southern US on I10 which until you get to the West Coast is a pretty flat interstate. I will say that Alabama Mississippi and Louisiana must have the worst roads in the world as I10 is terrible going through the Delta.
I found the six hours driving per day I could handle comfortably as I was solo for this trip except for my lab who is willing to take any trip with me. My departure each day would depend on the rush-hour traffic. Six hours usually put me at my next campground between two and three in the afternoon, thereafter setting up for the evening I could relax eat a meal walk the dog and review the GPS's and computer maps of my destination the next day and watch some television to get weather reports and catch some news
. I hope you find this useful