I went back and skimmed through every page of the thread and it seems most people started noticing the change after 300-500 hours. Same thing in my case as I noticed the change just after 500 hours of use.
Like others have said, if you have the problem you will know it. I remember seeing dark scenes on my TV when it was brand new. I watched nothing but full screen content and most of it was blu-ray. I can specifically remember in National Treasure 2 where the character is in the dark with a flashlight quite a few times and the blackness of the cave really stuck in my head. All I could see was the bright glow of the flashlight. This is also when I noticed phosphor lag
What is interesting is that I no longer see phosphor lag in dark scenes. I also don't stare in awe at the deep blacks since they are now gray. When the set is showing a black screen my room is illuminated as though I have a small lamp turned on. I hardly believe that this is normal for a plasma since quite a few of the newer LCDs have excellent blacks. My HP LCD monitor has about the same black level as my brand new plasma.