I guess this poses another question. Does running the CRT brighter burn out the flyback faster than say 50% brightness? I mean, it makes sense that it would shorten the life a little bit, but is that an amount of time measured in an hour or two, or 100s of hours over the life of the machine?
Yes, but nowhere near as fast as it would the CRT.
Not only does the flyback have finite life under heat/load conditions, but the CRT cathode itself definitely has a lifespan.
a Flyback transformer can "theoretically" last forever if the internal materials dont break down, and its not under undue stress electrically and thermally. I can prove this fact as my 1994 Magnovox tube TV I had since new still has its original flyback, still working, and still has a picture as it did when new. But one thing it DOES have is good airflow!
Also with CRTs, the more you use the CRT and the brighter its used at, the more cathode material that gets burned away and oxidized. Eventually you will lose enough cathode material to cause a drop in emissions. Emissions drop ever so slightly as the tube wears. But there is a point at which it begins to affect the picture.
The first thing that happens is the corners get blurry/out of focus, and when adjusting focus, you can sharpen the corners but the middle goes blurry and vice versa. Then it get so weak that the image begins to "bleed" from oversaturation of the drive signal. The saturation point drops as electron emissions drop.
On color tubes, the first to go is usually red and green. But not always. You can tell when the emissions drop below 25% is when the color bleeds like fire off to the left side of images. Such as if you have a nice bright sun in a movie scene, the sun has a blurred fiery flare coming off the left side, and it looks "washed out". Thats the sign of a dead tube, again from over-saturation of the cathode drive from the loss of emissions.
When a color tube starts looking like that, that means your emissions are now lower than 25% of health.
black and white tubes (like our compacts) will "bloom" and look fuzzy around the edges, and it WONT sharpen up with the focus control. But the brightness level has a severe effect. Thats a weak black and white tube.