To burn an image into an LED TV it takes around 400 hours of the image being displayed constantly on the LED TV Screen.
CRT screens usually burned the image in faster but not always.
The length of time it takes to burn an image into a TV depends on the type of TV.
CRT TV's were really prone to image burn in and especially the CRT arcade screens.
I've been purposely trying to find this out myself with an old CRT monitor and I've had the same picture displaying on the CRT monitor for 6 months now and still it has not burned into the screen.
However I've heard that some people have had images burn into CRT monitors within a few weeks to a month.
So maybe it's just my CRT monitor that is really good at preventing image burn in.
LED TV's are still able to burn an image into them but it takes much longer and most times it takes around 3 months for an image to burn into an LED TV.
The image on CRT monitors burns into the Phosphors when the image is displaying without moving.
When you turn the monitor or CRT TV off then you should see the image burned into the monitor but so far I have not had that happen to this monitor.
But I do remember seeing arcade machines have the image burned into the screen so it can and does happen.