I suppose it depends on how much stuff you're putting together and how easy or hard your chosen case is to work around. Generally not long though. A practiced builder will have it done within half an hour (not including any OS installation) though if you're less experienced it might take a bit longer than that.
It took me a bit longer because I forgot to order one part and had to wait *facepalm* but once everything was together it was literally just slotting them in place, a bit of cable management (and more later if you like), hook your monitor and everything together and install your OS and drivers. Took me a few hours all in all
When it comes to system building, I'm a perfectionist. Depending on how many components are being installed, a typical build will take me an hour. It doesn't take long at all to assemble the system but cable management is something that I do like to take my time with to get just right. If I'm doing a water-cooling setup, then it could take an additional hour or two.