A bad monitor will look like a blue screen, black or blue or green lines across the monitor which can be either horizontal or vertical and the bad monitor may even glow a faint green color.
A bad monitor can also go dimmer and dimmer and eventually may not turn on or there may be purple spots or other colored spots in the corner of the screen or anywhere else on the computer monitor screen.
In some cases the change of color of the screen or the lines can be from a bad video card or even from a bad VGA cable if you use VGA.
A loose VGA cable can also cause the monitor to have lines and appear as it's going bad or is bad.
You should only need to replace your computer monitor when it starts to go dim or has any imperfections in the screen such as lines, green spots, blue spots etc or has failed.
In some cases lines on the computer monitor screen such as green lines can be due to a loose or bad VGA or other input connection.
There have been times that I thought my computer monitor was going out but turns out it was just a bad VGA cable and sometimes it was just a loose VGA cable.
Simply unplugging the VGA cable and plugging it back in tighter helped it.
I only replace my computer monitors when they start to go bad or go bad.
I usually get 5 years and sometimes 10 years out of a computer monitor.
The computer monitor I'm currently using is around 3 years old and is an ONN computer monitor that I paid slightly less than $100.00 for at Walmart.
I expect it will last another 5 years or longer before it goes bad.
I also have some older LCD monitors such as an 17 inch Dell LCD monitor that is 18 years old and was made in March 2006 and I paid $2.00 for it at a thrift store.
And I also have a 17 inch LCD monitor that was made in March 2005 which is 19 years old that I got for free as it was given to me.
As long as you treat them right most computer monitors will last quite a few years before needing to be replaced.