I picked 45 minutes, but I really don't know. Slapping new parts together doesn't take me too long, except for the motherboard mounting. That takes forever, once that's done it goes quickly and I'm only slowed down by difficult drive bays which are hard to reach with screws so I drop them.
When having to pull parts from an old computer, or even from multiple computers, then it takes fricking forever.
To actually make a working computer, I set aside at least an entire afternoon, for OS install and installation of core applications.