To be blunt I think you need to be technically excellent and publicly recognised as such before hanging up your shingle to do this kind of work. Once you have people asking for you to print their negs then you worry about how much to charge and how long it takes.
If your customers are happy to pay what you ask then all good but if they baulk, you can probably then think about how to optimise process and/or whether the time you spend on a neg is reasonable.
I remember Thomas Keller, a famous chef of French Laundry talking about the restaurant kitchen. He said in order to produce meals out of that kitchen, you need the minimal input of labor and food to cook the dishes. In other words, for a restaurant to be profitable, the kitchen has to be efficient. As you can tell, I love to cook. I see parallels of working photographer's darkroom and a commercial kitchen. Both require technical expertise to be efficient and artistry to producing a pleasing product. All cooking techniques and darkroom techniques should be second nature if it's a profitable business.
"Photography, like surfing, is an infinite process, a constantly evolving exploration of life."