I've been working on mine for two years. Actually, my trainer, my massage therapist, and I, have all been working on it together - we've done a great job so far.
My expectations of what I want my shoulders to be capable of is a lot more athletic than the average person. But, I had made great improvement in range of motion, external rotation, and pain free movement within four - six months.
Shoulders, when healthy, are amazing joints. They are capable of an incredible range of motion, but they tend to be weak if not properly developed. Most modern humans have ruined their shoulders by sitting at a desk all day, and some of us have also done more damage by misusing them in athletic endeavors.
The good news is that humans are amazing, adaptable, healing machines, and much of the musculoskeletal dysfunction that people deal with is preventable and fixable. But you should find a good trainer, PT, and/or massage therapist to help you fix it.
When I started two years ago, I sometimes found it difficult to raise anything more than a few pounds to shoulder height. Last week, I held an 80-pound barbell fully extended overhead and walked the length of a basketball court, and the week before that I did 60 overhead squats with a 55-pound barbell. Amazing improvement, but it took two years of effort, and a lot of help to get there.