HDTVTest OLED burn-in test complete: No burn in after 6 months, 20 hours per day.
No burn-in at all, after 3740 hours (187 days at 20 hours per day). TV is an LG 65" E8 that Vincent bought himself.
One good point he made about burn-in on in-store demo TVs is that those TVs often are set up in such a way as to never get a pixel refresh cycle. The TVs are turned on and run all day, sometimes playing the same content over and over again, but then at the end of the day they are hard shut off via a power switch at the power bar so they never go into standby mode. Standby mode is necessary to engage the pixel refresh cycle.
FWIW, my LG C8 is now over a year old and has no burn-in either. However, I don't play any video games with HUDs for extended periods, nor do I leave news channels playing all day long. Our usage alternates between mainly broadcast and cable TV, Netflix and Prime Video, and Blu-ray.