What in gods name convinced you to put a 1000W PSU in a 100W PC?
Anyway, I used to keep CPU/mainboard combos for around 5 years in my gaming PC, with a midterm GPU+RAM upgrade.
Then hand the hardware down to my parents, where another 5 years of office computing are easily possible, after a PSU and fan swap, if I want to be perfectionist with it.
These time frames are only likely to get longer, with consoles no longer pushing graphics complexity as massively, as during their last generation, and Moore's law seeing diminishing returns, and hence the increase in speed of computing devices being reduced year over year, compared to the past decade.
Of course, if your demands change, you could quickly outgrow a quadcore machine. I know a few buddies who render a lot of stuff, and for them the quadcore is very much a limitation, and they're considering more or less seriously getting something truly big for the single socket - 15 core CPUs can't come soon enough for them. But since that's a completely different world, compared to the PC you described, I doubt that this is very relevant.
For the time being you will be I/O limited 99.99% of the time. I would guess that hardware decay is more likely to force you to upgrade, than actual lack of performance.