For both LCD and Plasma televisions, it is not so much the amount of time as it is the amount of use. Generally, for any tv set, the average life is about seven years.
It depends on how much you use your television. Other factors have an impact on the lifespan of a television as a whole. Such factors as if the T.V. is placed near a heat source, such as a furnace,heater, vent or fireplace; If the T.V. is inside of a cabinet where it gets hotter, it can have a shorter lifespan than a T.V. that sits stand alone where air can circulate around it. Heat is the number one enemy of electronics.