Howdy, Fry. As Alky mentioned, flight time on a tank of gas varies greatly among the aircraft. With regard to fuel, I usually take off with 100% in fighters (possible exceptions include the P-47N and the P-51). The way you can check flight time available is to use the E6B in flight. Press the "Esc" key to bring up the clipboard map, then click on "E6B." It will show how much time left you have at your current throttle setting.
I find the E6B mighty handy also in letting me figure out when I should start heading back to base and for managing my fuel on the way back if I cut it too close.
Some aircraft can take drop tanks. If 100% fuel doesn't give me at least 30-40 minutes, I tend to take drop tanks, too (like in the Typhoon) if they are available for the aircraft. You just have to remember to jettison the drop tanks when you are done with them, as they add a lot of drag (use "Backspace" key to select "DT" as your secondary weapon, then press button 2 on your joystick for each drop tank you want to drop, then press "Backspace" again to get your normal weapon 2 back on button 2).
