Onan Generators are industrial type units (Even the RV ones) and how long you can run it depends on two things.
How much fuel is in the main fuel tank for the RV (The generator can only use the top 3/4 of that tank)
How much load is on the genrator.
Basically. Fill the tank at the "Last Diesel before parking" and watch your main dash fuel gauge from time to time... In my case (Gasser 70 gallon tank) I can go about 3 days.
Answer 2: Till you run out of fuel for the generator
SUGGESTION: if you do not have one, GEN-TURI
Without the Gen-Turi it only took my generator 2 hours to set off the CO alarm.. So Consider a Gen-Turi. (With the Gen-Turi I've hit 25 hours on a few occasions and no alarm).