Well, we've only been married 2 years, so not long enough to have anything that needs truly replacing. Dh's computer coming in is on it's last legs, but we can't afford to replace it yet because we purchased a laptop instead a few months ago.
Once the laptop is paid off, we will replace his desktop. We like having one of each.
So my answer... when it croaks, or becomes so extremely slow and obsolete that it's not worth anything.