One of the projects I'm most proud of is still running ten years later, and has processed over a billion AUD through it in that time, with very minimal maintenance. I recently consulted on it, and sure enough it's still ticking along nicely! The code is honestly quite good too, even if it is PHP (though in a very nice microframework we wrote on top of Silex: removed all magic that a lot of these systems relied on. No annotations!)

I haven't doing this forever (only 10+ years) but surprisingly I think a majority of what I've written is still running. Probably a fair bit will continue to run for a while yet too I think (again, surprising for CRUD web apps).