At minimum you need to stop every 2 hours and take at least a 15 minute break to keep from developing fatigue when driving.
I try to stop at least every hour if possible to stretch my legs, take a pee break and check the vehicle over while doing so.
I used to be able to drive 2 hours straight before stopping for a break but now I try to stop at least every 1 hour to an hour and a half.
You should also avoid driving more than 8 hours in one day so you'll stay alert and get plenty of rest.
Before you set out on a road trip make sure you get plenty of rest the night before so you'll be well rested and alert when driving.