I think it depends on the project and the programmer. Code I wrote for a Nuclear Power plant is still there. Ditto for the code I wrote for the steel mill. And you can't find ATM machines w/ my code in them easily, but you see them from time to time. Code I wrote to go into the DoD's CAC card is still there, even though the hardware went through a new Rev a decade ago, so it surprises me to say this, but "yay, java?" Some of the code I put in Firefox to support TLS 1.1 & 1.2 is still there, but most of it got refactored out a while ago (and thank $DEITY Brian(?) replaced the old libpkix library that had been in Netscape navigator and then Firefox for about 20 years.) Much of the POTS network has been replaced since the 80s, but I'm told there are one or two switches I contributed to while at DSCCC are still around.

But on the other hand, my 1993 1 character patch to the Linux kernel was replaced in around 96 or 97. I hope to whatever benevolent Supreme being exists that the crap pyth*n code I added to Second Life has been replaced. No one still uses Palm Pilots or Handspring Treos anymore, so I doubt that code has much life in it. Virtually every web app I wrote is dead, but they were fun to write, so whatever. And the code I added to a couple of satellites is dead (though my ground station code is still alive.) I bet that some of the avionics code in the cockpit is hard to update as well.

So... it depends... my nuke plant code still has another decade probably and my old room-mate's anti-lock braking code will probably outlast us all. Embedded systems are probably more long-lived than the Facebook front page. Some are just hard to update cause you can't easily get to the machine, others are hard due to regulatory or compliance reasons.