Like most things on these cars there were several ' editions' .
The reason your description has the " or so" in it is because the glow plug light time is supposed to be influenced by the "temperature of the engine and ambient air " through a sensing circuit .

" When the glow system is switched on , each glow plug is subject to a current of approx 20 amps ( peak pluse of approx 40 amps).
Under the influence of increasing heat , the inherent resistance of the glow plug increases and will limit the current to approx 8 amps. "

" After a glow period of approx 20 seconds a heater pencil element temperature of 900 degree C / 1652 degrees F will be attained, after approx. 50 seconds the max temperature will be 1080 degrees C/ 1976 degrees F. "

Safety Shutoff
" If there is no start following indication of ready - to -start condition , the glow current is interrupted by the safety shutoff, after 90+20 seconds for model year 1978/79, after 50+10 seconds for model year 1980. "

Sometime around 1981 they took out the coolant temperature sensing and only have the ambient air sensor.... but in the description of the fault sensing indicator....as when one or more pencil heating elements fail .... it says :

"A fault in the preglow system is indicated by the preglow indicator lamp not lighting up when the key is actuated in position " 2".
In the event of unfavorable tolerances of the pencil element glow plugs or of the Reed relay (d), response of the fault indicator only after two pencil element glow plugs are defective is allowed"

".... If the balance of the magnetic fields is interrupted by the failure of one or several pencil element glow plugs, the Reed contact will close and the electronic unit (b) will be activated.

The preglow indicator lamp will switch off immediately and will therefore not light up when preglowing starts. "

That last sentence indicates that the light and the power to the glow plugs are not tied together. The glow plugs can be heating even though 1. the light ( by being off or flashing) is trying to indicate system maintenance is needed ( more than one glow plug has failed ) ...or the light is burned out.
This also makes sense in terms of default planning by the engineers... would they have a system which could cause the car to not start due to an ' advisory' light system being broken.. or would the logic flowchart give the last word to the vehicle operator and operation of the key ? Notice that when I quoted the time for glow plug continueing to heat up that I was using the lower more conservative time... the 50+10 seconds... when I said " up to a minute" ....