When I first got my Car I thought that the Glow Plug light indicated how long the Glow Plug Relay would stay on.
Later I found out that is not true; the Light is only a guide to give you some indication when you can attempt to start + the bit of a diagnostic feature. The Light does not control the Relay.

On mine the light is on about 10 seconds and if the Key is kept in the Preglow position the Relay remains on for about 20 more seconds. 10seconds + 20 seconds = 30 a total time the Relay is on until it times out.
If you want to Preglow longer; cycle the key and start over.

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84 300D, 82 Volvo 244Gl Diesel