Failing screen display on old iMac

I have a pre-Intel iMac, bought in 2005.
The screen display is now littered with around 8 very fine, vertical coloured lines - the turquoise and pink ones being the most visible.
It is my impression more are appearing at a quicker rate now - the first one was there for several months before another one followed.

I intend to buy a new iMac sooner rather than later, but I wonder if anyone can suggest how long this display might last before it gives up - could it suddenly fail completely, or is it likely to just peter out, with more and more coloured lines.

The screen also lacks brightness when turned on from cold, though this is not a serious problem once running.

This was a declared fault in a second hand Compaq I had. I put up with it for 6 months or so, I don't remember any significant additional degradation during that period.

It will get more and more lines.... I replaced mine after the third line, and a half-hearted attempt to get Apple to fix it for free, as it is a well-known issue with certain 17" white iMacs and the warranty was extended for it. But in truth I wanted a new iMac anyway, so I just replaced it. It still does sterling service as a spare TV/cookbook in the kitchen, albeit with increasing numbers of lines.

A friend of mine still uses his for video editing, although it is actually hard to see any screen at all now. He has grimly decided not to replace it until it actually dies. It refuses to do so.

Thanks, that is useful to know - It is one of those 17" white iMacs.