Quality is as important as quantity. Getting great coaching will cut down on the quantity, but regardless, most people, even with the best coaching and very hard work, will not rise above 5.0.
-Robert
Finally someone got it right. There are tens of thousands (if not millions!) of players who have played tennis for decades, hit hundreds of thousands of balls and countless hundreds of hours...yet they remain at 3.5 or 4.0 levels at best.
Why?
Because it is not so much the total time spent, but the technique being practiced over that time. You can serve ten thousand serves using a frying pan grip, standing facing the net, swinging leading the elbow through the contact phase and never, never, never, never, (I don't have enough 'nevers' memorized!), become a top level server!
You can hit a million forehands and backhands and if you roll the racquet, flip the wrist, open up your hips too early or don't develop a repetitive swing pattern within the parameters of a proper swing path, then you will not only not be able to hit more effective shots consistently, but you also won't be able to defend more effective shots consistently...and, at higher levels you must do both.
So, all of you, forget about total hours spent and focus on learning what you need to be spending time on. If you continue to revert back to those patterns you developed as a 3.5, well, guess what...you will remain a 3.5 player.
I often tell my students: "If you hit 3.5 level shots, you will always be a 3.5 level player." You won't spontaneously evolve inadequate shots into prolific shots just because you spend 4000 hours hitting such inadequate shots. However, if you spend 4000 hours working on the best techniques possible, you not only will acquire and become familiar with such methods, you will begin to evolve into a far better tennis player.
Not too many 5.0 players stayed at the 3.5 levels for very long...and it has very little to do with "talented athleticism"...(although, it doesn't hurt!). I have had players who clearly were not as 'gifted' become top level college players in highly competitive tennis areas, (southern Calif., Phoenix, AZ,).
Quit counting hours and start studying tennis. And then start practicing within those patterns.