When on a long road trip you should stop and pull over for breaks at least once every 2 hours to stretch your legs, use the restroom and get gasoline for your vehicle.
I always stop every 2 hours to every 1 hour and 30 minutes for a break especially if my legs are cramping up as it's nice to get out and walk around for 15 to 20 minutes and use the restroom and then get back in the vehicle and hit the road again.
Stopping more often is okay too and if you need to stop more often to use the restroom or stretch your legs then do so.
But as long as you're stopping for a break every 2 hours for 15 to 30 minutes at a time then you're doing okay.
You should also never drive more than 10 hours in a day because you can get driver fatigue and then lose your concentration on the roadway.