Review web site Rtings.com has performed an extensive burn-in trial for LG's 2017 OLEDC7 OLED TV models, and reports that the TVs have shown signs of permanent image-retention, or burn-in after only 4,000 hours.
Rtings have used six OLED TVs, running them from January 2018 for 5 hours each day at a time followed by one hour when the TVs are turned off. This cycle was run 4 times a day. One TV was at maximum brightness, and the rest at only 200 nits. Some played CNN, some a gaming title (FIFA 2018) and some of the TVs played regular TV channels. The TV that played CNN had a clear logo and static bar burn-in images, and the same goes for the FIFA game (but only slightly). The regular TV programs did not suffer from any burn-in.
LG Display claims that its OLED TVs last 30,000 hours without burn-in. It may be that the tests were very extreme. I can only say from my own experience that my 2-years old OLEDB6 does not suffer from any image retention.
Earlier this year, only four months after LG installed 69 OLED TVs at Seoul's Incheon Airport it was reported that the TVs suffer serious burn-in. LG later replaced the problematic OLED TVs at the airport's Korean Air Miler Club Lounge with LCDs.