When giving advice on writing, Elizabeth Gilbert (Eat, Pray, Love; Big Magic) takes the approach that this vocation is a calling, not just a job:
…if you are serious about a life of writing, or indeed about any creative form of expression – that you should take on this work like a holy calling. I became a writer the way other people become monks or nuns. I made a vow to writing, very young. I became Bride-of-Writing. I was writing’s most devotional handmaiden. I built my entire life around writing. I didn’t know how else to do this. I didn’t know anyone who had ever become a writer. I had no, as they say, connections. I had no clues. I just began…
Some writers may not need that level of devotion. Some.
But it’s fair to assume that you won’t have a shot of becoming a great writer without devoting your entire life to the craft.