Till it fails. Displays are at risk of proprietary nonsense. Older is likely better.

As long as it works for

Until it stops working?

Usually until the judge tells them to take it off, why?

I've been on laptops for the past 20 years, exclusively for 15, so whenever the laptop is replaced. But before that, I only replaced monitors when they died. And those old CRTs were built like tanks.

The only exception is when I upgraded my old 16" (or was it 15" ?) to a 19". My old monitor became a secondary screen for background stuff such as music player and IRC.

Until it breaks, or I obtain media which it cannot display. Actually, I wouldn't discard it in that case, it would just become my second monitor.

I was using a cheap+old HDTV until the power circuitry started to fail. It would still turn on, but only after unplugging it for a while like some sort of annoying timing minigame.

I got a dirt cheap monitor as a replacement. I accidentally ordered the slightly* wrong one (couldn't cancel it, then had a stuck pixel or internal dust(?) show up and messed up by getting a replacement rather than just a return), but even still it is a marginal improvement (with some downsides, like less inputs).

Still using a 1050Ti as well (and no reason to upgrade anyway), so I don't see any upgrades unless someone wants to give me a Polaris card and maybe a hand-me-down OLED screen with a bit of burn-in. I don't know anybody though (and don't expect it where I live), so it's very unlikely.

* The better version was about the same (and even a penny cheaper) but had HDMI CEC (so better power support... not that what I have supports it, but maybe it means newer/better electronics) and slightly better refresh rate