Worth reading this piece looking at how long it may take to "break in" to screenwriting--along with some other general rules.
whenever I’m asked, my own `general advice’ to any – and all – screenwriters/filmmakers, always includes, as 3 general `rules’:
1) Write 10 feature screenplays. It takes about that long to master feature screenwriting.
2) Always be writing 2 different screenplays. An expensive one and a cheap one.
3) Try writing the same film story: as an `expensive’ one – and, as a `cheap’ one.
Frank Darabont [A Nightmare on Elm Street 3, The Blob, The Fly 2, The Shawshank Redemption, Frankenstein, The Walking Dead]: “There are potentially more talented writers and directors than I, working in shoe stores and Burger Kings across the nation. The difference is I was willing to put in the nine years of effort and they weren’t.”