Rallying: ~17 balls a minute are hit by each player when calculated after 1 hour of hitting with my friend (this number involved watching some old rally video that lasted an hour). Playing 3 hours a day (which really factors in as 2.5 hours due to breaks), 5 days a week; you will hit approx. 12,750 balls a week. With this figure you will have to hit for ~3.92 weeks to reach 50k. So it will only take around 4 weeks to accomplish hitting 50,000 balls if you were to use my weekly hitting schedule.
Give me a racquet and I'll find a way to win!