enough boost is when you have 3 dimple marks in the hood from the valve cover nuts..
Mine goes a few days in my camper- a few lights, coffee grinder and a bit of laptop use.My inverter is an 850W from Home Depot, and will run a small angle grinder in a pinch.Ideally, make a spot in the car you can charge the batt when you are driving anyway.
Pharhaps this one, An 80 Amp-Hour battery would become fully discharged in 20 hours with a 4 amp draw.