If you want to see how long your glow plug system cycles, when cranking the car after dark, turn on your interior courtesy light
and cycle your glow plugs. The courtesy light will dim. The glow plug indicator light will stay on about 10 seconds. Hold the glow cycle on. In time the courtesy light
will brighten again and I believe there is a single click that is barely audible. This indicates that the glow cycle is over. You can time it and then you will know. I got this from a post on the Forum long ago, maybe from Leathermang. I also agree with Hatteras that syn oil helps in cold starting.