This is very basic coz I'm no techie, but I did live in a solar powered house for a while and that kinda rubs off.
Your car battery has a certain amount of 'amp hours'. A typical battery might have one hundred amp hours of electricity stored in it. Smaller batteries will have less, and larger more of course and there are different types of batteries such as deep cycle ones which you're gonna have to ask someone else about.
So lets say an average battery has 100 amp hours. For a 12 volt battery 12 watts is one amp. So a 25 watt globe, (which isn't all that bright) is nearly 2 amps. So running a 25 watt globe for half an hour will drop your battery's juice by one amp hour, leaving you with 99. You can work out the math from there. For example if you were running 3 50 watt globes for say 5 hours each night that would drain your battery real quick. (4 amp hours+ each for 5 hours = 20 amp hours times 3 globes = 60 amp hours in one night)
There is a complication though. Unless technology has changed a lot in the last 15 years, a car battery starts dying pretty quickly if you let it drop my lower than 80% full capacity. You can only let this happen a few times before the battery loses it's charge permanently and becomes useless. And also if you let the charge go down too much the voltage is effected also. So it is important to only use so much and recharge it regularly.
If you are setting up your camp for weeks at a time you could look into getting a small cheap solar panel. Or a generator or get it wired to your car. And/or a much bigger battery.
I think the smallest set up we lived with was two 6 volt batteries in series with 250 amp hours and two solar panels to charge it. It was plenty for just lighting the small house we had. The larger house we lived in had a series of 6 2 volt batteries but I can't remember how many amp hours in them. Again though we only used it for light and the two solar panels were plenty to keep us charged up also.
Hope this isn't too confusing.