We all have different tolerances. We will string together 3 days in a row of 500 plus miles if we have to. Usually do. Our final day this year was 650 miles to get home. We had "horse to the barn" syndrome. :)
However, there is no "fun" involved at all. The second and third day are painfully long.
If you're making time, I'd shoot for 400 mile days. If you want to feel leisurely, 300 mile max. We had a string of 250 mile days this year and that is addicting if you can afford the time.