The issue of incorrect aircraft endurance has been noted by the SEOW community. In fact, as an example, when we ran a large campaign recently that included the Solomons maps we found that the Betty endurance was seriously low, as juri noted above. Since our mission lengths were 3 hours in that particular campaign, we really needed much better modelling of fuel consumption in the aircraft involved. For example, it was well known that Zeroes had extreme endurance compared to Allied fighters of the time. HSFX consequently released a mod that corrected the endurance of the Betty and several other aircraft to historic performance. In the end, we could fly from Bougainville to attack Henderson, do some recon and return to Munda within the 3 hours limit.
Note that "historic performance" does not mean that you take off and fly at full mix and 100% throttle for 6 hours. You still need to fly at economy settings to get proper performance, and that is a whole new ballgame for most online pilots. Personally I would like to see this issue corrected for all aircraft in IL-2, but it would take a lot of work and would be lost on many people.