Becoming a writer isn't as hard as many believe it to be. It's more of an art form than anything else and requires practice to perfect your craft.
As such, the best thing anyone can do is start writing now—even if all they're doing at this point is keeping journals or taking notes on what happened during their day.
I tend to write using my lucid dreams as a muse. My imagination supplies me with an endless supply of possibilities. I don't have any trouble coming up with ideas when it comes to fiction stories because there are so many different things out there in our world today that could easily make great inspiration material for something fictional.
Suppose that's not your cup of tea. Try writing about your memories. Things you've personally experienced, give them a little twist, a little razzle-dazzle to spicy things up. If nothing else works, try creating some character studies from people who interest you. The important part here is just getting started! There will come times when words won't flow like water off a duck's back, but that doesn't mean we should stop trying altogether;
So pick up that pen or even use your cell and text your ideas. Just keep moving forward until you find yourself once again lost in the wonderland known as creativity.
The next time someone asks you how long it takes to become a writer? You'll know exactly what to say: "It depends on how good your imagination is."