How long does it take to get burn-in when watching widescreen content on a CRT TV?

[CRTguy]

I like watching widescreen movies and I like viewing them on CRT TVs but have been hesitant to watch them very much on my CRT TVs because I'm concerned about the image burning into the center of the screen and getting that widescreen burn-in. Does anyone have any idea how many hours of watching widescreen content on a CRT TV it would take to cause this type of burn-in? This is assuming that content is being watched at normal brightness and contrast levels.