How long will the batteries last when hooked up to this light?
First of all, you're ignoring that the inverter requires 12 volts. Two AA batteries in series is only 3 volts.

Second, the 5mA is the lamp current at high voltage. At 12 volts, the current will be higher. Using the 8 watts, at 12 volts, it would be at least 0.667 amps (probably a little higher due to inefficiencies in the inverter). Let's assume 1 amp.

If you use 8 AA in series to get 12 volts, at 3000mAh, you're looking at 3 hours, probably less as the voltage output drops as the batteries discharge, its unknown exactly how low it can go before the inverter shuts down.