The best of the plasma makers seem to go to 99.999% active sub-pixels.
Not sure on your screen, but on a 50" Pioneer that works out at about 10 full pixels or 30 sub-pixels befroe they consider it a problem, although there are often exceptions if the pixels are close together.
Clearly a lot of people consider it to be a problem way before that number of faults and it is always worth talking to the retailer that you bought from rather than the manufacturer.
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