It checks everything that has changed since last build and it changes only what is needed, I’d say 30 seconds or less normally. When updateing, you can end up updating all of the packages on your system at once, which of course is dependent on your network speed / sometimes compile time, but for me it can take around 10-15 minutes in that case. On a slow network, with a laptop CPU.

But you get atomic rollbacks so any breaking changes can be rolled back just by rebooting and selecting a different build.