You don't say how much experience you have, usually this will/should determine the "hourly" rate you want to make. It's too hard to tell you how many hours per client.. and I never reveal my hourly rate in my flat fee. My flat fee consists of all services and there's a sliding scale for the practice to grow. Once you have that number in your head then all you have to do is determine the flat fee by using the hourly rate and adding costs for that client. I have had some clients that only required a few hours a week to ones that took about 100 hours a month.. really depends on the practice. $30 per hour as your goal rate sounds about right if you have experience.