You will need to upgrade when what you are running does not match the performance you want, there is no other answer aside from some rough guesses. If you don't care about dropping quality settings the existing hardware will last longer with acceptable FPS in games. As a very rough estimate games seen to take a jump in requirements every 3 years or so where a mid range card from 3 years ago will need to drop settings to maintain FPS in current games. Of course there is no way to tell the future if some new engine will be out in a year or some game you want to buy.
The cpu does not care about resolution, it makes little difference.
As for gpu possibly yes but it depends on your expectations. If you buy a gpu that meets your FPS needs at 1080p it should last about as long as a gpu that meets your FPS needs at 1440p. However the 1440p will need a much higher tier gpu to start with.
You will need to upgrade when what you are running does not match the performance you want, there is no other answer aside from some rough guesses. If you don't care about dropping quality settings the existing hardware will last longer with acceptable FPS in games. As a very rough estimate games seen to take a jump in requirements every 3 years or so where a mid range card from 3 years ago will need to drop settings to maintain FPS in current games. Of course there is no way to tell the future if some new engine will be out in a year or some game you want to buy.