The half lifes for flat panel displays nowadays are in the ten thousands of hours or pretty much on 24/7 for 15-20 years before you would see a picture dim. Short answer - much, much longer than you will probably keep the tv or else before something else may break- i.e. power supply short etc..
HT- Panasonic TH-50PZ800U 1080p plasma, Panasonic PT-AX200u LCD projector, Elite Vmax Electric 100' screen, Pioneer Elite SC-71 AVR, Paradigm Titan Monitor v5 fronts, Paradigm CC-190 v6 center, Paradigm Atom Monitors v5 surrounds(x4), HSU research VTF-2 sub, Bowers + Wilkins ASW610 sub, HTPC/Gaming rig, Sony BDP-S550 blu-ray, Uverse HD-DVR, Roku 2
Bedroom: RCA 40: LED, BD-player
Den - Vizio 20"1080p LCD + RCA BDP
Don't believe everything you read on the internet - Albert Einstein