I live in Los Angeles so it's never too cold here. Maybe I'm getting too worried too soon. But I wait until the glow plug light goes off and then crank. Takes a good 7 or 8 seconds of trying and trying then, turns on.

Question is, does letting the key turned after the glow plug light goes off help heat the engine for a faster start? Or should I turn the key back, and repeat process 4 or 5 times?

 

Are you saying that after the glow plug light goes off, it takes 7-8 seconds of cranking (turning the engine with the starter) before the engine runs on its own?

If so, that is not normal, several possibilities, first thing that comes to my mind is that the starter isn't spinning fast enough. Could also be an injection pump or timing issue.

What is the year of your truck? there are two glow plug systems, the newer style uses the resistance of the glow plugs to regulate how hot they get/how long they are on for. If you turn the key off, then on again, they won't cycle a second time as they are already warm enough for the engine to start. I'm not entirely sure how the early system works (its all in the stickies at the top of the forum though!)

 

it sounds to me like you have a weak starter. you do not realize the starter is dying, because they slowly go bad over time.
here is two short videos of a bad starter at 70 degrees, and a good starter at 40 degrees.
a bad starter at 70 degrees it takes 7 seconds to start, a new starter at 40 degrees only takes 4 seconds to start.
and this is with 490+ k miles on the engine.
click on the pictures to play the video.

bad starter:


good starter:

 

if cycling the key 4or5 times then cranking the engine makes it start, its a glow plug controller problem. you should test each of your glow plugs first...