"How Deep is a River?"
This question is so very situation dependent that it is, for the most part, unanswerable.
The first part of the response has to be, "How comfortable is(are) the driver(s)." If driving is comfortable and relaxing, an eight our driving day is easy and a twelve can be done. If you have a driver and a navigator, that can take away a lot of the workload of being the driver. If you have two drivers, then you can go on for as long as you both can manage. If it is a motorhome, you will have to stop for fuel and to change drivers. If you have more than two, you can sleep one while the other two are on the bridge.
We try to schedule eight hour or 3~400 mile days. We have done lots of 500+ and even a few 1K+ days. That is never a first choice.
I have never successfully used an online trip planner. Both Street Atlas (Delorme ~40$) and Streets and Trips (MS Disc) will be able to effective predict fuel and EOD stops. Planning is both fun at practical.
Matt