To start,
we booted up the title menu of a Blu-ray (The Fifth Element) and let it run on the LG 55EG9100 (a full-HD OLED) for about 20 hours straight.
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The Fifth Element is a good test case because the main menu has several high-contrast features—such as a checkerboard pattern—that stand out when burnt into a screen. And sure enough, after 20 hours of run time you could still see/read the menu items (Play Movie, Languages, etc.) and that big checkerboard, even after changing to a blank gray screen.
After turning the TV off for 10 minutes, we turned it back on, and discovered that a healthy portion of the Blu-ray menu image retention was cleared up. It seemed like it was all gone, and wasn't noticeable when watching other kinds of content. On closer inspection, however, it was evident that there was still some minor image retention left over.
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After a couple of hours of use we turned the TV off for the day and when we came back the next day all of the ghosting was completely gone—it was like a brand-new TV.