How much time will a 12 volt input LED DC run on my car batter continusoly ?

How much time will a 12 volt input LED DC run on my car batter continusoly ? I need this to make a light for my handycam .Will the battery consumption increase for an additional LED .

A) How much time will one LED run on full charge ?
B) How much time will two LED run simultaneously on full charge ? (I am asking this since i can calculate for 20 LEDS since i am going to use 20 LED lights to make the flash light for handycam )