We actually timed this exercise where I formerly worked at once.  The conditions are:
All components are packaged with the exception of drives (CD, HDD, FDD) and memory.  HDDs are already imaged with the OS - hehe 7 disk imager.  Mobo, video, sound, NIC, case, CPU are in the retail boxes.  All components within arm's reach.  Case cover is not required to be on due to possible troubleshooting.  Time stops when computer boots.  Boot screen displays all drives, but OS isn't loaded.
I got it down to 12 minutes.  We had one guy complete the task in 8 minutes.  Cables weren't tied.  Used a power screwdriver.  This was production on a large order at a small shop.  Yeah, you cut your hands, even on good quality A-Open cases.  I'd be slower with a cheap case.  
At home, I take my sweet time.  Only install the basic components to get a boot screen.  Fdisk, reboot, format, reboot, OS, reboot.  Then add sound, reboot, NIC or modem, reboot.  Run a couple of diag programs.  Proof the entire process.  Tie cables.  Install 8 screws in CD readers.
It's like assembling anything.  Concentrate and complete the task often enough with the same components, you can be quick.