From my experiences determining the duration of each task is highly important. Through past projects within industry, I have found that project managers are sometimes over optimistic about how long a task takes. Finally, when that task exceeds the amount of allotted days, there is debate about why this happens or why it happened. I always highly suggest considering adding 1-2 buffer days in case some roadblock comes up that prevents you from completing the task within the amount of days allotted. Sure, it extends the timeline but at least it provides a more realistic expectation on when something will get completed in addition to the possibility of being ahead of the timeline.