Depends what you mean by OK level; that's *very* subjective. To play background keyboard parts with occasional melody lines in a rock band, I'd say 3-5 years of studying the right stuff (take your standard classical piano lessons and don't try to pick things up on your own and you'll be completely unable to do this no matter how much you practice), and it scales up from there with the difficulty of the music. Of course, your prior knowledge will help, and the faster you learn in general, the faster you'll pick this up. Don't walk into it without being willing and able to make a serious dedicated commitment, or you'll just be disappointed.