Korean tech site and phone marketplace Cetizen tested OLED displays on the iPhone X, the Samsung Galaxy Note 8, and the Samsung Galaxy S7 Edge for 510 hours to measure burn-in as part of its ongoing iPhone X review. In all three cases, maintaining a static image for far longer than a normal use case was necessary to produce noticeable burn-in. The iPhone X took longer to exhibit distracting burn-in than the other two phones.
The site left the phone screens displaying a static image at maximum brightness for the test's entire duration. The iPhone X first showed signs of burn-in at 17 hours, but even then the image retention was not bad enough to be noticeable in normal use. The Galaxy Note 8 took longer to exhibit retention, but by 62 hours it was more significant than what was seen on the iPhone X, such that a general user could identify the burned-in spots on the Note 8 at that time, but not on the iPhone X.
By the end of the 510-hour test, all three phones had very noticeable image retention that could potentially be permanent. Cetizen did not report trying any methods of clearing the image up. In OLED TVs, retention can occur after several hours—especially with things like network logos on broadcast TV, or persistent UI elements in video games—but it is usually easily reversible with the help of image retention remedies included in the TV's software. But the OLED panels in phones are made very differently than those in TVs, so it's unclear how much the retention is reversible in phones. It could vary from device to device.