The first really big writing project I ever undertook was my dissertation. When I transitioned from being in serious research mode to actually writing the thing, I gave myself the goal of writing half a page on it, every day. This is slightly more than it sounds like, because footnotes, quotations, and citations didn't count here. So really, it was half a page of my own analysis, every day. A good day, a day so good I would allow myself to stop writing if I wanted to, was a full page. About 300 words of my own. Maybe that doesn't seem like a lot, but if I had written every single word right the first time, I could have finished in about five months.
When I started writing fiction, I made 300 words a day my daily goal. (Well, approximately. One notebook page, based on the size of my pages and of my handwriting, is about 300 words. So technically, my goal was one page a day.) Again maybe that doesn't seem like a lot. And really, it isn't. But if you write 300 words a day, every day, then you've written 109,500 words in a year. Manuscript length for the kind of fiction I normally write is about 100K words. So 300 words a day is a book a year. You can even take 31 and 2/3 days off - be sick, take a vacation, write 300 of the wrong words one day - and still write a book a year.
And if you know what you want to write, the physical act of writing 300 words doesn't take very long at all. Well, so long as you get all the words down in the correct order. And you never have to go back and cut any words. And all the characters show up for work every day. And there aren't any holes in your plot that you need to go back and fix. And. And. And.
There, of course, is the difficulty. And the reason that once I realized I want to try to make writing fiction my career, why my daily page count is a lot more than one page a day. Well, that and the fact that I like to eat. And live in a house that has heat, and light, and water, and the internet. Right now, I sell short stories. I am not at a place in my career when the money comes before the work - no advances - and sometimes, I do the work, and I don't get paid. Not because I'm selling stories to sketchy markets or anything like that, but not everything I write sells. Sometimes I have to trunk a story. But let's say I were to sell every word that I wrote. 300 words a day x $0.05 per word (professional rates) = $15/ day. $5475 a year. That does not keep me in a house with light and heat and water and food.
Oh, and for those who were wondering, that's actually pretty close to the median first novel advance.
So, like most artists, I work at a day job, too. I'm pretty lucky here - I like my day job, and it's close to what I do with my writing, and I only need to work one other job. When I write, I work on multiple projects at once, in the hopes of increasing the amount of money I can bring in with my writing.
But really, the above calculus - 300 words a day = a book a year = $15 dollars a day - is why I get grumpy with people sometimes. With people who think that art ought to take a certain amount of time to make, or else it isn't art. With people who think that because what I do is fun, or creative, or because I've chosen to do it, that I don't deserve to be paid for my work. That because I can, if I choose, do the work for my job as a writer from home, or in my pajamas, that my time isn't valuable, or that because I love it, it isn't a job.