The goal is to know how much I could draw on it, for lighting my work area while changing brake pads, for example.
It depends entirely on the size of the battery - In Europe it has to be printed on the label, in other parts of the world you'll have to look it up based on the battery code, which is usually a three digit number. The other numbers to look for are the continuous current capacity (CA), which tells you how much you can draw as a constant load, and the Cold Cranking capacity (CCA), which is how much is available for starting the enging - although this latter value won't be as much use for your intended purpose.
As a guide, a small car might have one of aroud 30-50 Amp-hours, with bigger cars being 100-120Ah, and truck batteries significantly bigger. Batteries for diesel vehicles tend to be bigger as they need more current to start.