Answer by Fareed A.S.:
From my experience, a plan in years or even months is only a rough estimate, its not really a plan. You could say its something you want to do. For a plan you need to be more specific. Example, Burj Khalifa took over 5 years to be constructed (6 according to the wikipedia page). I think the planned duration was 3 years when it started. Now how could anyone plan it “correctly” in years even with the original specifications and duration of 3 years? Best way is to add suitable details to the plan to the point at which you can easily identify the ‘tasks’ involved in executing the works. Depending on the nature of the project, this lowest level of planning could be in weeks or days or even hours. It should at least provide a sufficient amount of detail and ability to plan, monitor and control the involved tasks so that the bigger goal, the Project, can be successfully accomplished.
About what your teacher said regarding 30% of time, I am not sure if you have posted it correctly. But if you meant that only 30% of the available time should be spent on planning, its obviously not something many people would agree with. Planning takes up a fair share of the time, even when executing the work is being done. You can’t just first make a plan and then go to performing the tasks and not plan for the tasks coming up in the next week or fortnight and to address backlog.