As others have chimed in, most will all agree that you are always improving regardless of how many years you've played. But I think you're asking something more to the effect of "How long does it take an average player to reach something like an A level, or even possibly a pro level?" And I think it depends on the individual, and how much time he/she spends not only practicing, but how active and exposed he/she is to competition. Two really good examples I can think of are two Japanese pro players that I used to compete against and play with regularly. Naoyuki Oi went from a pretty good amateur player, to the no. 1 ranked Japanese pro player in only about 2 years time. One year, we're playing about the same speed, and then this kid just starts tearing everything up. The both of us spent about the same amount of time on the tables, but his 6 days a week was spent on playing tournaments all around the Tokyo area. One tournament a night, 6 days a week. Whereas my 6 days a week was spent at the same club playing buddies, and possibly one tournament a week.
The other kid that comes to mind, is a kid I remember as a 12 year old C player that had turned pro by the time he was 17. His father owns a pool hall, so when he turned about 15 his daily routine would be to come home from school, hit 'em for a couple of hours and head off to a tournament every day. Both of these kids not only played daily, but played in tournaments almost daily. Which not only exposed them to competition on a daily basis, but also exposed them to tough players (some pros) daily. Not only did they play lots of pool, but they watched lot's of it.
dave
