My dad got me started playing tennis around the time I turned four, and because we lived near a park with about twelve tennis courts, he'd have me out there every day for about three to four hours or more. He was my coach (he's played all his life, has been trained by pros, knows the game well) up until I turned ten when I started working with an actual professional coach. I had the aspiration to go pro I guess you could say. Heh, even made the newspaper a few times when I was little.
Anyway, when I started working with my new coach, it'd be about three hours with him every day after school. On days off (or summer break/vacation days) we'd spend up to six hours on the court. Never a day off unless I was too sick to walk or injured. Unfortunately, about two weeks after my 14th birthday, I severely injured my right knee and had to get surgery. From then on I couldn't train as hard, and so the amount of playing and the hopes for ever going pro went down. Now (I'm 18) I work with younger kids and coaching them, plus my younger brother, who is now eight, is serious about the sport, so I'm his hitting partner sometimes when HE has to spend four/five hours out there a day on the courts with my dad and his coach.