My good buddy, we'll call him Dan, has parents who bought a big screen TV just so they could watch all the 'excitement' of the OJ trial (I don't remember what he did...something about putting void stickers on dead bodies?).
When the trial was all said and done, we decided to watch a movie on it; Terminator 2. When the screen would flash white, or any other bright colour, all you could see at the bottom, burned in, was "Trial of OJ Simpson" a strait line and "CNN" in the corner.
Boy did we laugh.
Anyways, to answer your question of how long it takes phosphor burn to take affect, I'd say it takes the same amount as the OJ Simpson trial. Or possible less, I'll never know.