Most people have heard the 10,000 hours saying. That is, it takes 10,000 hours of practice (specifically, deep practice) to become world class at any given skill.
A study at a renowned school of music asked top teachers of music to rate violinists. It was found that the students who were rated "best" (meaning they would go on to have careers as international soloists) and current professional players had accumulated 10,000 hours. However, students that were studying to become professionals and were rated "good" (could be pro players, but not soloists) and practiced less than 8,000 and 5,000 hours respectively.
I think we can apply this result to tennis. The average amount of practice for a musician who would go on to teach was around 4,000 hours. To be a tennis pro, the PTR requires that a player is 4.5. Therefore, I'd say it's reasonable to assume that it takes around 4,000 hours of practice to reach 4.5.
The "Good" group could be pros, but not soloist. They had obtained just around 7,000 hours of practice. This is level of skill is about equal to a college player, or around 5.5.
So, if this is accurate, to reach 4.5 (4,000 hours of practice) would require 8 hours of practice per week, every week, for 10 years. Sounds fairly reasonable.
To be a 5.5 (7,000 hours) one would have to practice about 13.5 hours per week, every week, for 10 years. Also sounds plausible.
Unfortunately, to reach 7.0 (10,000 hours) would require about 20 hours of practice per week for 10 years. Sounds likely, but I don't think any of us will find the time for that.
http://expertadvantage.wordpress.com/2010/11/29/10000hours1/