Charles Stross lays out the miserable truth about the practical issues when you try to write fiction for a living:

Most people have a very romanticized view of what it is that authors do. Firstly, there’s a widespread perception that the workload involved is relatively easy — in modern western nations, the level of functional literacy is high enough that a majority of the population can read a book, and write (at least to the extent of thumbing a 160-character text message on their phone). Because there is no obvious barrier to entry as with music (where proficiency with musical instruments clearly takes practice), most people assume that writing a novel is like writing a text message — you put one word in front of another until you’re done. The skills of fiction composition are largely invisible, until you try to actually do it. Secondly, many people harbour peculiar ideas about how much money there is in commercial publishing — and when disabused of the idea that selling a first novel is a road to riches, they assume it’s because the evil publishers are conspiring to keep all the money to themselves (rather than the unpalatable truth — publishing commercial fiction is hard work for little reward). Finally, there’s the Lifestyle chimera.

In short: it’s actually work to write for a living. The pay sucks for the vast majority of fiction writers. You face all the risks of a start-up business, but the potential pay-off is lottery-odds unlikely to come your way. Unlike other work, creative writing can’t be done (for most authors) in a predictable regular way:

Putting words in a row is wearying work. When they’re flowing fast, I can sometimes reach a dizzying peak output of 2000 words per hour for a couple of hours — not in fiction, but in a blog entry or a non-fiction essay. I’ve occasionally had death march sessions in which I pumped out as much as 10,000 words in a day. But such Stakhanovite output isn’t sustainable; a 10,000 word day is usually followed by a three-day-weekend to recover from it. A more realistic target for a full-time professional writer is 500-1000 words of finished prose per workday, corresponding to about 1-2 hours of writing, 2-4 hours of polishing, and another couple of hours of thinking about what they want to say, and how to say it. Like anyone else, they need weekends and vacation weeks and time to do the housekeeping. 1000 words per day for a 250-day working year (50 weeks of 5 days a week) works out at 250,000 words per year — or two 320 page novels.

There’s one SF/Fantasy author who seems to publish a new book every month, but he’s extremely unusual. For most authors, one or two books per year is pretty good output.