This is one of those 'how long is a piece of string' questions I'm afraid.
There are many ways to establish yourself as a photographer, and everyone does it slightly differently. But without a doubt one of the best ways into photography as a career is the old fashioned way - by working as an assistant (which is what much of this site is about - have you read the articles?). Studying photography at a college or somewhere certainly helps too, but isn't crucial. Some of the very best photographers are self taught, or have learned while working with someone else.
Photography is hugely competitive, and becoming increasingly more so. Without a doubt, the rewards are there for you to reap them, but you need to make sure you can keep your head above water. You'll know I keep banging on about this if you've read many of the other posts or articles, but I can't emphasise enough how important it is to have good business and marketing nous. In my view, if you want to have a successful photography career (rather than necessarily creating photos that will go down in history), you'd be better off studying something like business first, or at least getting some real life experience of working in a business environment before you launch yourself into the photography world.... And while you're starting out it certainly doesn't harm to have a sideline in something else to keep the cash coming in as it can be a slow old business building up your client base.
Having said that, photography is the best job in the world, so if you think you have even a chance of making it, you should give it a try or you might spend the rest of you're life wondering what you'd missed out on.
Hope this helps
