https://arstechnica.com/gadgets/2018/01/it-takes-17-hours-for-an-image-to-burn-in-on-the-iphone-x-test-shows/

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.