Me being one of those,I have a TRS-80 4k Home computer.
My answer:forever!
Amazing. There are at least a couple posters so far who should organize themselves and start a museum.
I tend to distribute whole systems to family members if they still have everyday use-value. I've donated whole computers to charity, but not so many. And I've built just about every machine I've had since 1995 -- my last OEM purchase -- a Gateway tower.
The old Gateways were stripped of their "working" parts eventually, which were then sent to the recyclers. The cases got extended use to house my more recent creations, until I could see advantages to junking the chassis and related parts to actually "buy" a case from a reseller.
It's a lot easier to save the three-wire-and-plug assembly for old fans; I probably have 6-32 hex-head screws from my OEM machines of the '90s -- never throw away screws. But screws and other fasteners aren't the major disabling aspects of "hoarding" -- they don't take up much space, and they always come in handy.
I DO have a collection of fans: Panaflo's, Evercool, Delta -- some old Zalman OP-1 92x15mm fans which I suspect may still see active duty again -- but maybe not -- who knows?
CAT-5 Ethernet cables. USB A-B cables. AC Power cables. COAX-TV cables. Splitters. Filters. Audio cables of different varieties. Bezels, 5.25" bay plates. IDE HDDs (time to kill them and recycle). The SATA-II drives -- those are still useful. I certainly have spare SATA-III drives, don't I?
A couple G-bit Ethernet NICs. One old but well-revered sound-card.
For motherboards and processors, my holdings are not excessive. I think I have three motherboards in storage -- any of which could become the basis of my next home-server re-build -- including an EVGA 780i board!! The remaining two are Z68 and Z77 boards -- anticipating one more replacement project when Moms is more eager to give up her LGA-775 C2D.
I have too many spare kits of DDR2 RAM, though.
But mainstreamers don't do this stuff. They don't save screws, wires, cables, fans, HDDs.
They tend to think they can replace a desktop with a tablet, and may be disappointed. Anybody who hoards old tablets and cellphones has to be a bit nutty. A major implication of mobile devices that can't be customized: they're the substance of a throw-away or recycling culture.
If I'd had greater vision a decade ago, I would've started an electronics-waste-recycling company of my own. They seem to exist side-by-side with facilities of local government, and I suspect the two types of entities work hand in hand.