It varies. If you have a brand new large battery, you stand a good chance. If you have an 8 year old battery in your car, chances are less.
Below is my experience. If you are going to do this for one session of 7.5 hours, I think you stand a decent chance of starting the car afterward (depending on the car and battery). Bring jumper cables and park facing out for good measure.
I brought a spare car battery to a festival with an inverter and charger thinking I could charge DVD player batteries out of the trunk all weekend. I got 1 DVD battery charged and was part way through a second when the inverter kicked out because the car battery was too dead. That was a 300 watt inverter with a fan to keep itself cool, and it ate the battery.
At another festival we ran off the car battery and charged about 6 DVD player batteries over the course of 2 days, before the car battery died. That was a smaller, more efficient inverter. We did have to get a jump to get the car going again.
Going from DC > AC > DC again is just inefficient. Now I have an adapter than will charge DVD player batteries from 12V, with a DC-DC converter, which is more efficient. I'm expecting better life with that.