[old_tv_nut]

Since burn-in happens very gradually, this is hard to answer, because it depends on how sensitive your perception of it is. To minimize it, your viewing of letterbox images should be as small a percentage of the total on-time as possible. Of course, this means using the CRT more for 4x3 images, and shortens the life of the CRT. There's no free lunch.

It would be nice to have a widescreen source that puts average grey in the blank areas instead of black, but I haven't heard of one that does it.