12V @ 1.3 A-hr would be 16 W-hr. That site says the lights are about 5W, so based on that you should be good for about 16/5 or 3 hours. But it's not clear if the "4 - 6 Watts" is the average power or the peak power when flashing. If it's the latter then they'd last many times longer - how many depends on the duty cycle of the flashes.