My BH is an on again off again technophile and I normally inherit his cast-off (laptop) after 18 to 30 months. Usually my oldie lasts until his next cast-off becomes available; once in a while he has to go shopping for something new to provide me with his cast-off. I'm not a techie at all. I do word processing, surfing, e-mail and bits of sound file editing, so my computer requirements are not nearly as high as the leading edge of what's available. What was available even 4 years ago is more than enough speed and storage for me. I stopped using desktops 2 or 3 computers ago when we got a wireless router, preferring to be flexible about where I sit. That's also when laptops reached what I considered to be a price worth paying.

So I guess our combined answer is, "when we have to."

There's a charity in the nearby city that repairs/refurbishes dead computers for needy kids, which is where the corpses go.