I don't think there's any hard or fast rule to this.
I'd been learning ruby for about two years when I got into Rails.
I then read a couple of books, hung out on Rails forums, listened to Ryan Bate's Railscasts a lot, then got the chance to built my first web app.
Since then I've gone on to write some more complicated apps, but this original app has become a pet project and I rewrite it every year or so.
I am always surprised at how bad my code from twelve months ago actually is.
So,to answer the original question: it depends. Personally I needed about two and a half years experience from the point I first opened a Ruby book until I could produce anything decent in Rails.
What about everyone else?
It would be nice to hear other peoples' views.