Depends what light you have.. That battery claims to have 7.2 amp hours. Or you could write that as 7200 milli amp hours. If a light draws 500mA then just divide 7200 by 500 and you get your total amount of hours. In this case it would be 14.4 hours. Assuming a full charge and good condition battery.
Amps and volts are much more useful units than watts. For most applications that is.
