45 minutes/bike, give or take, depending on quirks of the particular bike -- 7sp kids' 24" bike or single speed bike, less time; commuter with fenders and rack, more time, same with higher end bikes which arrive in a more disassembled state and technology that's new to you like Di2.

How many you get done per day depends on whether you're also dealing with repairs and/or working the sales floor on top of just assembly.