The manufacturer will rate the life of the plasma and the rating is for half-brightness 3.500 hours seems a bit low to me. I could be wrong but I think current generation TVs are generally rated for 8.00 hours to half-brigthness.
Of course just because the manufacturer says 8.000 hours doesn't necessarily mean you will observe the same lifetime in use. Still the longevity should be long enough that that is less of a concern than other factors such as is it bright enough for your room, is the picture quality acceptable, etc.
Burn-in is a concern with CRT and Plasma but with reasonable precautions such as not leaving a static image for hours it really isn't something to be overly concerned about.