As others have said, it totally depends on the motivation level of the student, parent and the teacher. My child's piano teacher is a prefectionist, I am a perfectionist and so is my child. That makes the practice tough on both my DS and myself.
My DS needs me to schedule the practice time into his day as he is still very young and he also needs me to be around to help him during practice (although I am doing less of this as he gets older).
We set a goal of practicing for the same # of minutes every day as his private lessons (this is the expectation of his current teacher). We accomplish it most of the time, but we let things slide occasionally due to travel, sickness and overscheduling etc.
I like the Suzuki quote above on practicing. My child's teacher told him to practice on days of the week that end with a "y"!
DS also belongs to an "ensemble group" (not school or band related) which has classes every other week - this is more fun than work for him because there are a few kids there who are not at his level yet and hence they play easy pieces. But, he gets a lot of enjoyment and ensemble practice. I recommend ensembles as early as possible as they teach a lot of musical skills in addition to providing peer groups - this is so important for my son as we are a non-musical family and can only read books on music with him.
