Run it as long as you have fuel for it. They are designed for long run times.
Our A/C usually runs 18 hours per day everyday with some 24 hour runs for several days during heatwaves. You do want to keep from running the A/C if the generator begins to surge due to lack of fuel.
Don't turn on the A/C until the generator is warmed up (couple of minutes) and turn the A/C off before you turn the generator off.