Same reply as marzer. There's no display, keyboard or battery to go wrong, it's not portable so the chances of damaging it are extremely limited. That means I would probably keep using it until at least 2035 or even 2040.
And before you think that's impossible, keep in mind that I'm posting this from my mid-2010 Mac mini.
edit: and then five years later, since it's a beast of a Mac with maxed out upgrades, I sell it and buy whatever base model of Mac mini is available at that time and still have a load of money left.