I would agree that appropriate practice and lesson time are highly dependent on the child in question. I do, however, have general rules of thumb.
A longer lesson time is valuable only if the student can absorb and remember enough of the additional instruction to productively practice it between lessons. How much a student can take away from a lesson will, of course, depend on her interest and capacity, as well as attention (which is often developmental). My personal experience is that I and my siblings started with one hour formal lessons around age 8 or 9. I notice that the standard in many places these days is 30 minute lessons well into high school. Our children's regular lessons are 30 minutes, but they do throw in some 60 minute lessons here and there, which they appear able to absorb equally well.
For practice time, again, it depends on the attention, interest, and capacity of the learner. I've taken the same general guideline recommended for homework, which is 10 minutes per year-of-age-minus-five. (Or ten minutes per year of grade.) Obviously, if a child wants to practice more than that, and it doesn't have negative impacts on other aspects of development or family life, then I see no reason to stop him. With my very young or inattentive children, I sit with them for the whole (very brief) practice session. All of our kids like music, so they're on their instruments quite a bit--though not always in the form of focused practicing.
I want my children's experience of music to be both enjoyable and instructional, so as much as committing to taking lessons means committing to daily practice, it ought not to be at the expense of music as a delight and a vehicle for emotional/artistic expression.
