We have 2 drivers and my wife starts in the morning. We aim for 2-hour shifts but also like to be at the campground before 5 and preferably before 4. This often means only 3 shifts (I offer to change but she likes to continue). So, taking an hour for lunch, we get 300-360 miles a day.
We do our Florida trips straight down -- 3 1/2 days. Our west coast trips were more relaxed as we spent some time seeing places en route (and got held up by the weather).
I plan using Streets and Trips, putting in the start and far points, then looking for campgrounds at or before the 4:00 o'clock point. Then put in any mid-trip destinations.
We also like a bit of recovery time, in case of events beyond our control.