How far per day depends on lots of things. I'm retired, so time is not as important to me as it is to many others. I prefer the secondary roads to interstates, so that tends to slow things down. I like to stop in the early afternoon so I have time to set up & explore the area, particularly if I'm only going to be there for a day or two. My best miles per gallon happens at 57MPH.
With all that in mind, I prefer to do around 150 miles per day, will do as much as 250 if necessary, but have often done as little as 60 - 75.