Individual goal governs the length of the practice duration.
If one wants to play with good quality, talent and also depend how many pieces that one is practicing. In addition, how difficult the piece is a major factor in determining the length of practice time. A concert pianist needs to practice at least 4 hours a day.
Little John (5 years old) who just learns how to play Minuet in G by Bach, may need to practice at least 30 min a day. If wants to be really good fast, he needs to practice one hour.
Bottom line, it really depends on the goal and ability. There is no wide brush that fit for every scene.