when can an author quit the day job and write full-time?

A week after Cuts Like a Knife hit the market I had a neighbor ask, "when can an author quit the day job and write full-time?" I got the same question from two authors within days. Here is a quick glance at the numbers that helps anyone that wants to start a new enterprise count the cost - I picked a salary with an easy monthly number and then walked authors through the royalty process. Enjoy!