I replace mine while the "old" one is still useful - therefore for the last (since zx and ti99) how long!! I have given them to a good home . On one occasion I gave one to a school (it was far better than they had even seen) but it took a lot of persuasion to get them to understand it was a gift and not a scam!
My current old has win7pro and win 8 pro - quad core 3ghz. I use a more up to date one and when that gets a bit tired the older one will go.
60 yrs ago I know what it was like to have "more month than money" now I asked my undertaker if I can have a very big pocket put in my shroud - he said no!LOL