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As soon as I can get a 50%+ performance improvement for £300 or less, for the most part.
Though right now, I'm unsure whether to upgrade my GTX970 unless I get a 1440p monitor.
If you can't run the games you want to play at the settings/resolutions you prefer with acceptable frame rates to you, then that's the time to upgrade.
I suppose if you have money to blow then upgrade every new release.