I asked a group of people how long they thought a technician would take to do a full service on a standard sewing machine. I was amazed at the response. Several said 30 minutes. One said 45 minutes, Another said an hour. It was obvious to me that they had very little understanding of the work required.
In our classes, I challenge new technicians to shoot for an average of one machine every two hours. To do a quality job requires expert knowledge, concentration, and speedy action. Time is money. If the charge is $100, and it takes ten hours to do the job; you earn very little. With years of practice and expertise, a great technician may be able to average as little as forty five minutes to an hour. But even the best can face that occasional machine that takes a whole day.
If you have a bunch of Singer 221 Featherweights, you may average as little as twenty minutes.
If you have high end combo machines, it may be more challenging.
However long it take, the quality job is primary.