Why do you need 20 seconds? All you are accomplishing then (by waiting even longer after the light goes out) is running your batteries dead.
LLY's only need ~4 seconds even when its below 0 degrees. LBZ's and LMM's need even less.
contrary to popular beleif/assumption, leaving the key on longer than the glow plug light dictates (before cranking the engine) or """cycling the glow plugs""" more than once before cranking the engine,
DOES NOT DO ANYTHING beneficial on a duramax when the outside temps are cold. Remember this isnt a 6.5 or a 6.9 IDI...its a "next generation" diesel that doesnt really need any special operating procedures; GM designed the dmax to be driven by anyone who can figure out how to put the key in the ignition... the glow plug module is a lot smarter than the egg timer's they put on those older diesels.
ben