> Whereas my brain seems to be able to work at essentially the same intensity for as long as I want it to, up to 18 hours a day, and then I need a bit of sleep to recover.

Potential variable explaining this - what's your age? I could do this too in teens/twenties.

> How do other people here experience mental fatigue (or not)?

Just normal tiredness / distraction. Two days of actual full-on active pairing on something tricky and I just want to go to sleep after work.