Typically, I say that I can give you a pretty good swing in 5 lessons, a great swing in 10 and a phenomenal swing in 15.  That's 1 lesson per week for that length of time with practice in between lessons.  Considering it took me over 10 years to develop a swing I was happy with, it seems like such a small price to pay.  Unfortunately, many people won't wait that long.   It's not because they don't have time it's just that they want results immediately.

If you've played golf for 15 years and you still shoot in the mid to high 80's or 90's the first thing you have to realize that the golf swing you are using doesn't work.  If you played a guitar for 15 years wouldn't you be able to give guitar legend, Eddie Van Halen, a run for his money?  Now, I'm not saying you should shoot 72 every time you go out but after playing for 15 years shouldn't you be able to shoot in the 70's regularly?  Knowing that you have a golf swing that doesn't work is the first step to playing the best golf of your life. 

The second step to playing the best golf of your life, is knowing that if you decide to change your swing, it will be like starting all over again (but this time it won't take you that long).  Initially, changes to your game will cause you to hit some very poor shots.  This is because you're learning a totally different way to swing.  You are un-learning years of bad habits.  It is this critical moment in time that you must decide to press on.  No matter how bad your game gets ... the future is bright. 

The third thing you must do is practice.  You have to ingrain your new movements until they are second nature.  The best way to make changes is to do consecutive repetitions.  What I mean is you have to do the same new movement time and time again. If you do consecutive repetitions of the new movement you will master it faster than if you do old swings in between.  In the early stages it's all about making the changes.  The faster you can change the faster you will get better.