Typically a car battery is 50Ah - which means it can deliver 50 Amps for 1 Hour before it is flat.
Your laptop power supply should have a label which will give you an idea of the current the laptop draws - mine is 4.7A.
If conversion were perfect a 50Ah battery could supply that current for
50/4.7 = 10.6 hours.
But conversion isn't perfect - assume 70% efficiency and that gives you 7.4 hours.
That's for a standard car battery - if you have a heavy duty you may be able to get twice that.
Also I imagine that 4.7A is a peak current rather than continues so again you will get more.
So figure 7.4 hours worst case.
Also remember your car is charging the battery at the same time, or you are stopped and you are falettening your battery.
Cheers,
Nigel
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