I had 3 good glow plugs, and my wait light and duration of glow plug cycle was about 4-6 seconds in warm temps. Once all eight were burned out, i could hear the solinoid clicking open and closed in about 1 second intervals. (confirmed with a voltage test). After i replaced glow plugs, it went right back to 4-6 second cycle.
I'm sure this just adds to the confusion... Sorry 'bout that.