If you don't already understand rate of oil consumption, I suggest shutting down daily long enough to check oil level. You are probably going to get about three days running with a full tank of gas (if E-450) because of where the auxiliary fuel line taps the tank.
If you are talking weeks at a time, your will be getting into service intervals, requiring longer shutdowns for oil changes and air filter, spark arrestor, spark plug checks.
Mine usually needs oil topped up every couple days, running 6 to 12 hours a day while driving.