Nothing every completely disapears. When radio active material breaks down, scientists use the measurement of half life to determine the lenght of time it will take to decompose to a certain level... mostly to determine when they will be safe again.
The Half life a anything is recuring. So, your first half life is 60 000 hours. Then, whats left of that, goes through another half life. This can be a different time to the first. It doesn't actually break down at a specific rate. The second splits to 2 parts again and deteriorates again. Somthing to do with the breaking down of certain materials (i can't quite explain that). If it were an easy equasion they would just tell you it has a life span of 120 000 hours.
So, the second period (that being anything past the 60 000 inital hours) may break down quicker, or may take longer. It has its half life of its own. Basically it could be another 40 000 hours then leaving its other half to break down. Possibly being 20 000 hours. So in essence, by saying it has a half life of 60 000 hours doesn't mean it will last 120 000. Denpending on calculations, it will probably last a lot lot longer than that.
Thats all you need to think about. Sure, if your going out to buy 5 ft rear projector screen tv you will want it to last but somthing like a plasma, their sure to upgrade them pretty quick and a new one would only be a few years around the corner.
Its about the half life of the Plasma in the TV. No plasma... no picture. So, if your planing to go blind byso, you'll be fine. But if you want to "see" your images, youll need to buy another on in 60 000 hours.
Its exponential - hence at half way through the half life rather than having lost 1/4 it would be closer to 1/3. As for picture deterioration that i am unsure of, i just know half lives and how they work. But that would make sence - so after 10 years your picture may have deteriorated 25%, but much better to 25% after 5 years for the others.