I agree with both pek64 and SweetSilver. I think time management skills cannot necessarily be expected to simply arise in good time -- not in all children, not even with good modelling by parents. Sometimes a more active approach is needed, and sometimes a child's aspirations to learn or do something will exceed their current time-management skills in which case parental help is appropriate if the child wants it.
But I also think that the scenarios in this thread are not primarily about time management. They're not about the child forgetting to do the work, or leaving it too close to a deadline, or having trouble fitting the work into his busy life. They're about the child avoiding the work which would get them closer to his goal because the work itself is not intrinsically enjoyable. So to me this is more an issue of deferring gratification and doing grunt-work that is unengaging in order to reap the benefits of having done the work later. Like practicing scales to get better at playing Mozart, or doing abdominal crunches to get better at swimming. Or learning division of fractions to get closer to your goal of studying architecture at college.
To me deferment of gratification is to a large extent a maturity-based task. It requires a fair degree of abstraction to choose your future happiness over your present comfort. School provides the structure that removes a lot of that choice, so school children tend to have less trouble working through relatively unenjoyable basic learning tasks in the early years. However, because they lack the opportunity to grow through making such choices I think they tend to struggle when the structure of attendance checks and homework-grading falls away in high school and college. Maturity is required, but it is also developed through experience.
I think in order to learn how deferring gratification can be helpful in some situations, one has to experience in meaningful ways both the drawbacks of not doing so and the benefits of doing so. After experiencing both those outcomes in a variety of circumstances one can appreciate why sometimes it is good to defer gratification. When it's all just hypothetical, when you haven't experienced one or the other scenario, it's hard to make a mature informed choice. And with my ds and his intense perfectionism, I felt like he hadn't ever had the experience continuing to work diligently through the really uncomfortable (for him) place of not being able to do something easily yet, and realize the benefit that comes at the end. Once he had that one guided experience, he was able to assess his subsequent issues in light of his past experiences and make reasonable choices.
I'm also a big believer in awaiting readiness and meaningfulness, and I think that's the scenario school children often don't get enough of. They don't get to put grunt work off until they find their own reasons for doing it, and so they don't learn how satisfying it can be to learn things quickly and to completion when the task is intrinsically motivating. Unschoolers though can suffer the opposite fate: they sometimes don't get the experience of taking on tasks that aren't intrinsically enjoyable in order to open doors, build confidence and bring forth opportunities down the road. Not unless their parents help them find and work through those experiences.