The assembly doesnt take very long. Adjusting is an other. Depending on the complication also. A minute repeater can take a very long time to perfect to get the right tempo and strike. When I visited Patek philippe they talked about several months for a single watch. Also precision (competition chronometers) can be very demanding to adjust and used to be a seperate skill in itself. I'm shure that quick assembly and easy adjustment plays an important role in today's movement design as manual labour is expensive in Switzerland.
Christian