We drove all the way across the country when my dd was 4 months old, and we found that about 6 hours of actual driving time per day was all we could manage (and often it was more like 5 or less). It took us all day to get in those 6 hours, too. We'd start out around the time Lindy was ready for her morning nap, then stop after a couple of hours and let her get out and nurse and play for at least an hour, maybe a couple of hours, then start driving again. In the evening, she tends to get fussy unless she's being held or playing right next to me, and even if she's sleepy she often can't go to sleep easily in her car seat then, so late in the day we'd often try to drive but would have keep stopping when she'd start crying.
We have some relatives that are about an 8 hour drive away, and what I've been thinking we should try is to leave very early in the morning, before Lindy would normally be awake, so we can hopefully get at least a couple of hours in before her day really starts. That way we might be able to manage 8 hours of driving. If your baby can easily fall asleep in the car when it's his normal bedtime, you might also try making the last part of the drive be after he's asleep for the night.
One thing that might make the trip more pleasant for the adults: when you're ready to stop, make sure the first place you stop is somewhere you'll be happy to stay for a while - a nice, shady park or a pleasant sit-down restaurant, say. My dd hates to get back in her car seat again too soon after stopping, so we found it was a big mistake to think we could just make a quick stop at a fast food place or gas station, then head for a park or something afterwards. She'd wake up at the first stop (if she wasn't already awake), and then we'd usually be stuck there for a while since she'd want to nurse and would absolutely not want to get back in her seat any time soon.