When we were both working and needed to be somewhere, like getting home by Sunday night after a Saturday NASCAR race, we would do 500 to 600 miles in a day.
Now that we are retired we pretty much limit it to 200 to 300 miles per day, sometimes we will run a little longer if we both feel like pushing a little farther.
One advantage of our HDT is that long days are nowhere near as tiring as they were when we towed with a pickup.