How many hours will the 900-BBK work with EL devices?

Resolution: The 900-BBK battery backup is really designed for short periods of power outage time and is not meant for long hours of backup.  The batteries are 12 volt DC rated at 7 Amp Hours (7AHr) each, but under load are always discharging including driving the power supply circuitry, so the run time is not linear.  Also, in general rechargeable batteries should not be discharged beyond 50% so as to maintain a longer battery life, unless the batteries are rated for deep discharge / charge cycle functionality.

Running time for access control devices attached to the 900 series power supplies functioning only from the batteries is extremely variable, including depending upon the attached devices current draw at the selected voltage output and battery configuration:
Output: 12 volt DC (two batteries in parallel is 2x 7AHr = 14AHr rating) or 24 volt DC (two batteries in series is 1x7AHr = 7AHr rating)
Example: PS902 at 24VDC & 2Amp output:  7AHr / 2Amp = 3.5 hours maximum, but in practice probably would not run this long.

An example use is the PS914 with 900-BBK is expected to allow Von Duprin panic devices up to several hundred activations on fully charged batteries with intermittent power draw.
In other cases of constant draw on the power supplies, (2) 7 amp hour batteries if fully charged running  in series at 12VDC with a 1amp current draw could last possibly up to to 7 hours or more to reach the 50% battery discharge point.