I actually did this when we had a power outage for almost a day. I wasn't sure what was going on so I needed a way to run the TV for awhile. I used a 150W power inverter I have and used my '97 Toyota Camry (4 cylinder) to run the inverter. I only used it for about 1 1/2 hours but I let the car just idle out in the driveway and ran an extension cord back into the house. I don't know the wattage of the tv off hand but it was a 13" I have in the kitchen. As long as the engine is in good shape, the fluids aren't low, and the front of the car isn't obstructed (like having it nosed into a shrub or something else that would decrease airflow) no damage should be done to the car or engine.
As for how long I don't have an exact answer but the gas gauge on my 4 cyl Camry didn't even move. I would guess that the car would run almost a full day on a full tank (18 gallons). If I had to run it all day I would still shut it off every few hours to give it a rest.
P.S. Too bad that inverter couldn't run the 'fridge. We had to throw most of the food away..........after we had gone shopping the day before.
Of course the alternator couldn't handle that kind of a draw either.