I'm not sure if this is more a question about how far total, or how far is comfortable on any given day. Our first trip saw just a hair over 4000 miles go on the truck in a week, although some of that was at the destination.
We had to do over 600 miles in a day several times during that trip, and that kinda sucked, for us. It just felt like too much work, when it was supposed to be a vacation! We are now planning on exceeding 300-350 in a day only under extenuating circumstances, the kind that aren't 'just one more day here?'... lol. Part of what we really enjoyed on our trip was just relaxing at the end of the day, just the two of us, enjoying the day, the scene, and each other; we'd rather have more of that than less.
But, to each their own; having a family can make that harder to do. I work, for now, and the kids are all gone, so what works for us may not be feasible for others. We also enjoy the trip as much as the destination; having time constraints usually changes that...