Ummm welll it may not be so simple. There are diferent kinds of SLA batteries often with different design chemistries and charge/discharge characteristics. Find out who really made the battery. May not be who is on the lable. Get the performance information from them. Yuasa, Hawker, Interstate, whoever. Overseas? good luck. Put the battery on charge with the voltmeter attached to the batttery. Watch the voltage climb and compare to the charge curve for the battery. When the voltage has stabilized you are generally done. More charging just wastes power and can ruin the battery. With a really smart charger you will see the voltage hit a peak - full charge - then drop to a trickle or maintain charge voltage.

OK, now that you are charged, plug in the lights and the voltmeter. Turn the light on. When you have reached the end of discharge voltage or ~80% of full voltage or the knee in the discharge curve STOP and recharge immediately. You now know how long the lights will run. AND if your battery charger indicates full charge before full charge voltage is reached or after full charge it needs replacing.

I am not a real battery expert. Contact the maker, or one of the specialty stores mentioned in the light selection sticky like www.houseofbatteries.com