Depends on the site what you have the site for etc etc etc..
But I've always been fond of building it , forgetting it and on to the next thing.
What I'm working on now is a constantly changing ,morphing site that changes depending how you got there , where you are from and with constantly changing relevant content.
This is why for me , I'm quit quiet at the moment.
Secondly , why update and modernise one site , if you can update...1000s ;-))