Odds of Getting In
Grid size: big. Word count: lots. Difficulty level: SO hard.
Since crossword construction is mostly freelance, you could start doing it today. Whether your puzzle gets accepted, however, depends on multiple factors. Many puzzle editors are wading through a giant morass (source). They're looking for gems which display an original perspective and fit the guidelines. Before you submit, check out the publication's crossword requirements to see what editors expect for grid size, word count, difficulty level, etc.
How else can you learn what crossword editors really want? One way is to get feedback from your puzzles that didn't cut the mustard...the silver lining of each rejection letter. Or, you can find a mentor to impart their wisdom to you. Crossword blog cruciverb.com is a great place to get advice on a variety of topics, from publisher response time to a good clue for "khaki" (source).