It really depends on the length of the trip and what you're trying to accomplish. Most of our summers we will go stateside for one week early in the summer, usually either Portland ME or the Boston MA area, I tend to leave Friday night, make it to the border (about 4 hours) and sleep there for the night, this allows me to do the 4-6 hour drive the next day and be at my destination early in the day, do groceries and setup at my own leasure. On the way back we tend to get up early and get the 7 to 10 hours of driving done in one day. I'm on the road by 6:00 and home by supper with fuel and food brakes in between to keep the kids happy. On our longer second vacation, I tend to get to the border Friday night and then Saturday drive for about 8-9 hours or a little more than halfway - Mostly we end up somewhere north of Virgina for Vacation and spend 4-5 days then slowly head back with 3-4 day stops in between.
We get to see a lot and enjoy quite a bit of country while really only doing one or two longer stints. My 3 and 5 year old are used to this and great to travel with.
My vote is I wouldn't do more than an average of 450 miles a day on the longest of travel days as I think not being a professional driver i'm not used to it. If I was retired, i'd do 200-250miles a day max while averaging no more than 55mph, a nice a relaxed pace (not bad for a 30y-o!). I love getting up early and getting some good driving in and being done very early in the afternoon - seems like the towing is always easier when the air is colder, the coffee is flowing and i'm perky!