Interior Lights Function

[TX '90 ZR1]

Any of you guys or gals know how long it should take for the interior lights to go out with a door left open?
Mine seemed to be taking a long time to go out, so I timed them. They stay on for 15 minutes and then go out. All other functions of the interior lights seem to coincide with operation as described in the FSM. There is no mention that I can find in the FSM that addresses the above question.
Also, does anyone know if the time can be adjusted? 15 minutes seems like a long time.