A study orchestrated by K. Anders Ericsson who looked at musical prodigies found the common denominator for mastery and success: 10,000 hours of practice. “The emerging picture from such studies,” says neurologist Daniel Levitin, “is that ten thousand hours of practice is required to achieve the level of mastery associated with being a world class expert--in anything.”10,000 hours is 5 years' worth of full-time work, i.e. 8 hours a day, 5 days a week, with 2 weeks of vacation per year.
Thank freakin' goodness you don't need to be "a world class expert" in order to write a saleable book. Most of us can't write full time, never mind doing it for five years without seeing any financial return.
However, if you consider someone who is making a living with their writing, and who does do it full time, five years isn't very long--only five years to becomes "a world class expert". And really, I would agree someone who can make a living at writing for five years has valuable expertise they could share with others.
If you're serious about a writing career, you should probably be striving to become a "world class expert", but in order get there, you need to be saleable enough to get the luxury of time to practice writing for 10,000 hours.
How many hours did you write for today?
Every little bit counts and helps, so never beat yourself up over that number; just make sure you're making steady, daily progress. It's going to take some grit to get to 10,000 hours. Keep chipping away.
This is why the first rule of writing is "Butt in chair, hands on keyboard." If you want to be a writer, you must write.