I've ridden as hard as 93% over 2.5hrs (normalized power), yesterday I did a trainer ride at 80% for 2.5hrs (mostly 82%). I'd concur with heathpack that 80% is probably a good target for longer duration rides, once you get over 83% you start getting into sweet spot, and that gets a little harder to maintain for too long (and if you see the chart on this page, it suggest sweet spot for up to 3 hrs https://fascatcoaching.com/tips/how-to-sweet-spot/).

If you're observing power as you go, I'd make sure your head unit is showing normalized power (and intensity factor), it'll give you a better idea of the effort as you go along so you don't blow up too soon