I 'hit' my limit at the end of trips. A couple years ago I went from Breckenridge, Colorado at 10am to Ft. Collins at 7pm ish, then all the way to Salina Kansas (all motels were full) (half way through Kansas) at around 2am. I was fine until the next day, driving home from KC I couldn't drive anymore half way through Missouri.
Same thing happened on the drive up to BC this winter 4pm-Midnight (Stl-Salina) 5:30am-8pm (Salina to Rawlins WY) 5:30am-1am (Rawlins WY to other side of the mountains in Washington) 7am- to 10am Seattle, walked around, then went to the outlet malls, got to the border at around 4pm. Physically couldn't drive anymore once I got to Vancouver.
So I don't get tired, I just physically shutdown/quit. This happened in South Dakota after driving around with the windows down in the Black Hills. Wind in my eyes for long periods of time = sleepy. So I stopped in the grasslands and took a nap for an hour and a half, was perfectly fine for the rest of the day.