For a CRT screen to burn an image into the screen it usually takes at least a few months to as long as a year in some cases.
The same image would have to be constantly displayed on the CRT screen for at least a few months before you got a slight bit of image burn in.
I tested this with an old computer and old CRT monitor and had the same picture on the CRT Screen for a few months and after a few months I noticed a slight burn in on the image.
But it took a full year of that same image being constantly displayed on the CRT monitor screen before it got fully burnt in.
It was an old CRT monitor that I wanted to test with as I was curious about this as well.
Some older CRT monitors and CRT screens got image burn in sooner because the Phosphors were not as good as the newer CRT monitors and screens.
The Phosphors on a CRT tube also eventually wear out and mess with the image being displayed and usually the phosphors on the CRT wear out before the TV or monitor actually fully quits.