Here's a post by NVirkkula, who is organizing it, explaining the rules. There aren't much of them really, this isn't an ultra-professional league, just something we do for fun!
TBK-Light Endurance Series
The game: Rfactor
The mod: Endurance Series Mod 1.2
10 Races, Provisional calendar
2. Vanport (Portland)
3. Elkhart Lake
4. Brands Hatch
6. Old Ring
9. Laguna Seca
10. 1 heure du Mans (http://games.on.net/.../7871/LeMans_24 ... or_Rfactor)
1x mechanical damage
1x tyre and fuel usage
Auto-Clutch allowed (no other aids)
Short cutting rule: two tyres on the track all the times, if broken, slow down significantly. If overtaking or protecting the position by shortcutting, you have to let the other driver to get the position. Fair-play required all the time.;)
The GT-cars from the Endurance series season. Reserve your ride in this thread! If you want to use your own team, post a download link in this topic of an easy to install package (i.e. zip-file that you just install in x:\rfactor-folder).
Schedule for the race day:
Free practice 30 min before Qualifying
Qualifying 30 min
Warm-Up 10 min
Race: max race time 90 minutes
The points system:
F1 2010, 25-18-15-12-10-8-6-4-2-1
If you're interested go to TBK Light and sign up, then go look for the "TBK-Lite Endurance Series" thread in the Computer and Video Games forum.