Hi Dan,
I will answer your question but bear in mind I mostly do clocks and only work on my own/friends watches and am nowhere near as fast as the other guys. For a simple mechanical wristwatch without date etc around 4-6 hours. That is if it is just a strip, clean, lubricate assemble and I have no distractions/problems/repairs. Don’t get hung up on how long it takes, at the start you need to focus on doing the job well so you end up with the cleanest, perfectly lubricated watch. As with anything, the more you do it the quicker you will get. Don’t take shortcuts, these will cause you more problems than if you had just done it right in the first place. Eventually you get into a rhythm and if you do this regularly enough you will find you can turn a watch around pretty quickly and have confidence in your work.
Paul 
