I try and keep it around 400 but it varies. Days where there is lots to do are closer to 300. Days when it is just a freeway run it is in the high 400's. Days when it is all back roads are in the lower 300's. So 6 to 7 hours.
We always plan stuff to see along the way. Even if it is just a quick stop for the world's largest cow. I plan the basic route then she hits the internet and finds those things. We stop for a real lunch at mom and pop diners most days. We also book nice hotels and make sure there is good food nearby.
It's different from a guy trip to the Gap. Hotels, food and things to do are important. I joke about the casinos but she loves them, so we hit them. They are quick stops.
As big as this trip was I maybe should have planned a day off somewhere. Not sure where as we were in the middle of nowhere most of the time. We did plan one shorter day to do laundry. Just hit a good laundromat and bang it out.
We did decide at the last minute to do two 500 mile days the last two days. We were in Fargo. 1000 miles to home. Doing three short freeway days seemed like a waste of time. The first 500 was a long day as we spent 2 hours at Princes house. That was a 12 hour day, but not bad because of the break at Princes house.
Unfortunately Prince wasn't home.
Start out small, 3 or 4 days. Find yours and her grove. You will have to negotiate stops. My wife prefers more frequent stops although the last day I talked her into 130 mile runs between stops and she seemed to be ok and recognized the advantage. Frequent stops does not make it better it just drags it out when it is just a freeway run.