Hmmm. 3-5 years, give or take or bar x or y is the rough standard established for me.
That's been since 2016 though so not a rule as such, more finding the best to fit an expected target or whatever... nm going forward.
Suffice to say I've had limits as to what I'll pay for what outcome vs preferences, at least re the core PC build as the centre of whatever else is added. Currently that's some £2000, give or take, to secure solid 3440x1440 performance with what was, I guess, the cheapest '4K' GPU last gen. As that resolution is where I prefer to be, even with a 4K option rn, I'll aim for much the same average performance for the resolution next time around plus whatever for new peripherals etc as and if necessary. I could say I won't pay more for a GPU than £1200 or £400 for a CPU or (especially) more than £175 for a 32Gb RAM kit (ouch) but that's only cos that's been about my limit so far... who knows what the future might hold though...
Disposable income isn't always great for me (being disabled unfortunately) so I tend to go for bang for buck options, nearly as good for way less aso (like the 6800XT and 7900XTX vs the 3080 and 4080 when the latter were £500+ more) and per my gaming library/backlog being a mix of AAA's indies and the strategy games that consoles don't cover, up to a decade old. Looking ahead I can see that RT and DLSS/FSR aso will actually be a compelling feature for me more than before but I think AMD are still doing well enough in that respect for the price difference.
The very recent news re FSR4 has been gratefully received as it gives me some leeway in time at least before looking again at upgrades, incremental or otherwise. Likewise the possibility of a rerelease for the 5800X3D to better my old 5800X for a while.