I think this question will depend on a variety of things, how much money someone needs to cover expenses, how much time they have to put into the business.
For me, I started my own business so I could work less... so to live comfortably, I only need to work about 50 - 60 hours a month. I used to work a lot more than that when I first started, but spent hours on my business and finding clients... now I don't need to do so much of that, so at the moment I am currently working 3-4 hours per day, which is nice.