Well, it depends on the trip. If it's driving from Houston to Marfa, and we just want to get there, that's sensibly doable in a long day - changing driver every two hours, with a lunch break half way - leave at 0730h, arrive at dusk.
If I want to explore on the way, I'd break it up into 3 or 4 days.
For long trips we assume 170 Km (105 miles) per day. So if we're planning a 50,000 KM (30,000 mile) trip, it'll take about 295 days. This is based on our experience of three "big trips" on which we averaged 170 Km/day on all three.
If we know the road will be difficult, we ask advice from locals - perhaps our slowest average so far has been 109 Km (68 miles) in Eastern Uganda, which took 4 days...
At the other end of the scale (and not driving our 4x4) we've driven with a lot higher average in Europe. One day, travelling from Southern Germany to Belgium we covered the first 200 miles in exactly 2 hours. This was simply to get from A to B. A more leisurely trip was from Belgium to London via Scandinavia - 5000 Km (3000 miles) in 16 days, so more like 300 Km (190 miles) a day, staying in lots of places and visiting many more.