I see some logical cutoff time lengths based on two factors. Tires and fuel.

1.) Does the race time require a gas stop? That would increase the cost because folks would want quick fuel systems and that is pricey. There is also a safety factor that one must consider when handling fuel quickly around a hot race bike.

2.) Does the race time require a tire change?. Again folks would want captured bearings and spaced front brake disks to decrease tire change times. Tires and bike modifications would increase the price to race.

In summary, I'm for longer endurance races. I would race a 45 minute endurance race. A 4 hour race would be really cool.