Ubuntu systems at Google are upgraded every LTS release. The entire process of upgrading can take as much as 4 months and it is also quite expensive as one reboot or a small change can cost them as much as 1 million dollars.
Link to Original Source
"Well, it don't make the sun shine, but at least it don't deepen the shit." -- Straiter Empy, in _Riddley_Walker_ by Russell Hoban