Sucks no one's responded to you. Hope you're still listening.
I can only respond to our 1 hour races but may be something you can gauge.
I have an ESR310PV motor and would be running on fumes at the end of a 1 hour race (actually ends up being closer to 1:08 because they throw the checkered flag at the 60 minute mark so you still have to finish your last lap). Never ran out of gas but one time I got back to the pits and then the bike died on me due to no fuel. That was a stock tank. I upgraded to a Clarke 4 gallon tank and now have no issues running the parade lap and a 60 minute race. My guess would be an oversize tank would get you pretty close on a 2 hour race if you didn't run a parade lap or practice lap. I still think I'd pit on my last lap just to be safe. If you're running a stock tank, you may want to carry a 0.5 gallon reserve bottle and just see how far you can get before you run out and then run your pit stops on that lap.
Hope this helps and happy racing!