80% of all businesses fail, regardless of the market demand or the quality of the product. They almost all fail for one single reason - Poor Business Practices.
Anne Geddes failed at business, she went bankrupt and had to sell the off all the rights to her photos. But she is an amazing photographer. She went bankrupt because she spent more money than the shoot would make for her.
Those posters here saying to make a business plan are 100% correct. I have been a Marketing Consultant for 25 years, and I can tell you that regardless of the business you start, if you do not have a defined market, products for that market, and a plan to acquire the customers you need to generate enough money to pay all your expenses, taxes, and then give yourself an income sufficient enough to live of of, you WILL FAIL. It is simple economics.
Sorry to say, but I also agree with the people who say the quality of your photography is not the standard to which you decide if you should go pro. I would love to say it, but the reality is many people have been very successful in markets where the quality of your photography is inconsequential to your earning potential.
Case in point is businesses who do Santa Photos in the Malls. These companies can bring in insane amounts of revenue, and deliver the crappiest photos ever, and people will wait in lines for hours to get them. I did a Business Case for one, just for fun, and I was shocked at the money that can be made doing it.
As another example the most successful burger company is McDonalds, but they are far from being what most people would consider a "Pro" burger. They make their money on shear volume and on Brand.
So to the OP, forgetting about the word Pro here, you want to know when should you start a business. My advice is as others indicated make a business plan so you know who your market is and how you will target them. Start small, while you have a safety net with another part time job. Build your business and market yourself. Make a Brand for yourself, and as your business grows along with your reputation you will find you will reach a tipping point where you can't work 2 jobs, and need make the decision. If you believe based on your Plan that the market exists that if you had more time you could get more clients and make sufficient profit to derive an income to live off full-time, take the leap.
You don't need to be full-time to be Pro, you just need to be in business, and being in business means making a profit.
just my 2 cents.