A pro writer told me he was at a conference and one of the panelists (working screenwriter) was asked if he had any advice to new writers. "Quit." After the laugh from the crowd subsided the writer explained, "It's damn hard work. The odds are against you. Spend time with your friends and family. Exercise. Do anything else if you can."
The average pro takes ten scripts and ten years to break into screenwriting. Saw Terry Rossio and Ted Elliot speak at the Nashville Conference (great event) in '09. They gave themselves ten years to break in. Wrote ten scripts they never showed to anyone before they had one they felt was ready. They broke in the business in five years -- well ahead of schedule -- and went on to write Shrek and Pirates of the Caribbean.
The excellent book THE OUTLIERS by Malcolm Gladwell has a chapter discussing the early years of The Beatles. Grinding it out hour after hour in seedy Hamburg clubs is where the developed into pros. Studies show it takes ten thousand hours to master something -- anything.
So if you write 10-20 hours a week (assuming you have a day gig) that's ten years of consistent effort to hot those 10,000 hours. If you can go at it fulltime -- average working year is 2,000 hours -- you can get there in five years. Are you writing this many hours?
If someone asks you, "How can I become a screenwriter?"... Now you know what to tell them.
Ten years. Ten scripts. Ten thousand hours. Or you could just tell them to quit...