How long a new tennis ball will last? Primarily it depends upon your skill level, play style, game type, court type, and whether playing with ball machines or not?
Usually in professional matches balls lasts shorter for 2,3 hours or 7/9 games. Regular ball with moderate hitting lasts more hours/games. Practice balls with ball machines can survive for days.
Simply when ball becomes fuzzy, can be squeezed easily, have very low bounce, sound changes like flat ball, then you should change ball.