Having your own business is not a guarantee of income. In fact, it is a risk. But this calculator will give you some idea of what is possible.
Note that if you do zero marketing, you will probably get few if any sales. It also depends upon the niche you are selling to and the quality and number of your designs and products they are offered on. I have some clients who get 2 or 3 sales per HOUR while others only get 1 or 2 sales per MONTH. My recommendation is that you start out trying to generate at least $1,000 per month and you put in enough effort (time) to make that happen.