While every writer dreams of the day their writing will pay all of their bills, most writers live paycheck to paycheck and search for that next writing piece that might put them ahead enough to make ends meet completely.
So, how do you know when the writing is paying the bills? You calculate what your average monthly bills are for three months in a row, then subtract that from how much you have made from your writing in the same time span. If your writing income minus your bills leaves you in positive numbers, then you are starting to do good. If you have a significant amount more from writing than you are paying in bills one of two things may have happened: you might be doing really good, or you might have forgot a bill (you remembered the mortgage payment, right? Cost of food?)
Ideally you want your writing to consecutively pay the bills month after month with some left over for one to three years before you start to think about quitting your day job to write full time. And then you should have enough set aside to live at your current standard of living for at least one to three years after you have quit your day job just in case your writing hits a hard bottom after you start doing it full-time.
So, what does it take? Hard work, and a little-- no, a lot of luck combined with good planning for when is the right time to test your wings. It is always better to glide down slowly than to fall, and with luck you will take off and fly.