This is my overview of the times I did the enduros in, whilst winning them.
GVS A-Spec - F1 Stealth - A little over two hours
GVS B-Spec - X2010 - Don't really remember. Probably about 1:20-ish
Laguna A-Spec - R10 Stealth - A little over two hours
Laguna B-Spec - Nismo Skyline R-Tune - Don't really remember but probably at best 2:45
Indy A-Spec - X2010 - Little bit over two hours
Indy B-Spec - Furai - Around 2:45
Suzuka A-Spec - Not done yet
Suzuka B-Spec - Fully modded Gallardo - a bit more than 6:30 with rain starting from about lap 100
Those are just rough times for cars that can win these races.