I was curious, so I set up a eight hour race with a handful of AI cars to run while I was out one evening. When I got back they'd all run out of fuel and were parked on the circuit. I was half expecting them to have refuelled by the "AI repair" option, but no, they just ran their tanks dry.

Do AI cars not pit at all? Or if so, how do I enable whatever setting is required to make them pit in long races?

not yet.

They don't pit yet but they will in the final S2 version or in future patches for the alpha