Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Burn-ins usually happen when a constant image is displayed for prolonged time periods in high brightness. Takes much longer at that brightness level.
 
It does turn off in a handful of scenarios if it helps. But at that brightness level you should be safe from the burn-ins.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.