What's up with screen burn?

I read another post somewhere regarding the possibility of screen burn on the vacuum-flourescent display.

I'm curious what kind of time frame you'd need an image up in the same place in order to produce a visible problem. For instance, one of the vis modes is a simple text display of the artist/album/year/time. It was my favorite display mode, but now I'm afraid to leave it in that mode for fear of the horizontal line getting burned in.

Is it like the old pong-game problem, where you'd need to leave the thing on for a week before it starts to hurt the screen? Or can there be visible degradation after a shorter time period?