The honest answer? Probably never. Yes, a few people do make it to the top. E L James of "Fifty Shades of Grey" is the most recent proof of that phenomenon. But for every E L James there are a couple of thousand ordinary writers. A handful of them will be doing quite nicely. Able to quit the day job and concentrate on writing. A bigger group of upper mid-listers will be making enough money for an annual vacation somewhere pleasant with their family or friends. But for most of them, their royalties will vary from a celebratory dinner out, to a new laptop, or possibly both.
The average advance for a new author is about $1000. A nice amount indeed, and worthy of celebrating, but nowhere near enough to leave the day job for. Especially when you consider how difficult it is to sell more than one or two books per year to a print publisher, and how infrequently authors earn much more than their advance.
Digital publishers rarely pay advances but they do permit an author to publish four or even six books per year. Since e-book royalties tend to be much higher than print (frequently around 45% instead of 8%) it is more than possible for e-published authors to earn that much in a year.
So my advice to you is to enjoy your sale. Celebrate being a published author. Cherish the fan letters. Perfect your craft, making each book better than the previous one.
But don’t quit your day job yet.
Helen is available to line edit and/ or content edit fiction and non-fiction. Rates on application.