How much longer will an i9-14900K be good for high-end gaming?

The 14900K is still a very capable gaming CPU, so I wouldn't be planning an upgrade yet. Several more years seems reasonable, but there isn't a reliable expiration date for high-end performance.

A future GPU upgrade could expose a CPU limit sooner, especially if you're chasing very high frame rates. The answer also depends on which games you play and what frame rate you'd consider acceptable.

Resolution matters here. At 1440p you'll sometimes be GPU limited and sometimes CPU limited, depending on the game, settings and frame rate target. At 4K, the GPU generally has more work to do, so the CPU may be less likely to hold back the frame rate.

That doesn't mean switching to 4K makes the processor faster or guarantees a GPU bottleneck in every game. Pick a monitor for the experience you want, not as a way to extend the CPU's life.

I'd wait until a game actually feels slow before worrying about a CPU upgrade. Check performance in the games you care about after a GPU upgrade, including frame rate dips, not just the average. If lowering graphics settings barely improves frame rates, that can be a clue that something other than the GPU is limiting performance.

Overall CPU usage alone can be misleading because a game may be constrained by a few busy threads without using every core fully.

There's also the separate question of stability. Some 14th generation Intel processors have had stability concerns, which isn't the same thing as becoming too slow for games. Keep the motherboard BIOS updated and check the manufacturer's guidance if you run into crashes or instability.