Why does screen go black after a few seconds

SOURCE: My Asus PG221 monitor suffers from frequent issues

I'm afraid this looks like an electronic fault. Some component, most likely a capacitor, has degraded, is overheating and going out of spec.

You just might be able to squeeze some more hours of work - but the monitor is still likely to drop dead any moment now - by rigging a computer fan on the backside to extract hot air from the inside.

Possibly, the problem might have a noise factor - try adding a ferrite shielding on the cable (unless one is already there, as it often is) and see whether the thirty minutes' grace period extend somewhat.

A probably unsatisfactory solution might be to drive it at a lower resolution, so that the more relaxed frequency constraints do not stress the component so much.

When the capacitor finally blows, you might be able to recognize it and have it replaced; it's likely a US $1 component and is probably in the LCD driver section. Perhaps, just perhaps, you could already be able to see it with the naked eye looking for a component that looks "wrong" - a small cylinder that looks bloated, with electrolyte leaking, whitish powder around the base, etc. But you would still need some solder work to get it replaced - and, if it's a SMD component, chances are it can't be done at home.

Sorry for the bad news...