If a device is rated at 12 V/80 mA what does that mean as far as the length of time that it will draw 80 mA?
It will draw the 80mA current for the period it is on, ie, has power applied.

If you are powering it from a battery, that's when the time comes into play.

For example, an 8 Ah battery would power the device for 8 Ah / 0.080 A = 100 hours.

BUT


A battery's storage is often quoted under various conditions which might not be continuous discharge (and are often given as multiple values at different currents).
You might well also find that the holding voltage is lower than 12V, so that as your nominal 12V battery's voltage drops below 12V the device still holds the door open, it will depend on the strength of the spring or other door closer.
Battery charge capacity varies considerably with age, charging history and battery technology
If the device is a simple electromagnet, its current will vary with voltage (basically linearly)
Be aware that batteries and devices in general might give nominal, maximum, minimum, typical, average or "specification" values on the label without saying which.  And caveat utilitor, nobody really expects to see the manufacturer's Highway Mileage or even Urban Mileage on their car.  For a fire door, make sure your system is fail-to-safe (and tested!) and be clear about which responsibility is whose.