How long (in hours) will a 6 Amps LED light last if powered only by 12V 18Ah Lead Acid Battery only
Battery capacities are measured in Amp-hours. You know how much current your circuit draws, so this is pretty simple: 18Ah/6A = 3 hours.

That assumes a constant current. Without a regulator, the voltage will drop as the battery discharges, so the current will drop too.

The capacity will also change depending the how much current is drawn, and the temperature. Here's an example from a Panasonic battery datasheet: