As long as you have a reliable mains power supply; indefinitely.
A UPS will only run on the batteries when the mains fails.
If you are using a standard off the shelf, AC to DC 12 volt power supply.
The A620 is rated at 6 volts; that means you need a 12 volt to 6 volt regulator.
Half your power consumption will be wasted through the regulator as heat.
If you can purchase a 6 volt power supply even better.
A very very rough estimate:
your camera consumes 3 watts.
your battery is rated at 84 watts.
switched mode PSU are 60% load efficient.
you are virtually not applying any load to the UPS.
In essence your camera will represent a no load to the UPS.
Be aware that most UPS's output square wave not pure sine wave.
You may experience parasitic oscillations, if the loading is not correct.
End result, your UPS and power supply may self destruct.
Quite honestly you would be better off purchasing a 6 volt gel battery and use a continuous charge, regulated charger.
That is the approach they use in the electrical servicing industry, especially in fire alarm and security panels.
