Often it can be hard to understand the profound difference between effort and duration.
So, let us take a moment to examine this distinction and why it is important.
During the estimation and planning process, the effort and duration are determined for the planned tasks.
Effort is the amount of work units required to complete any given task. Effort may also be referred to as man-hours, man-days, man-weeks, man-months, or even man-years. In order to determine the task duration, the effort required to complete the task must be determined first.
Duration is the calendar time required to execute any given task. Duration is measured in hours, days, weeks, months, or years. Duration can only be calculated once we determine who will perform the task, how many people are going to perform the task, and whether they are available to perform the task at a reliable level of availability.
Once you have an effort estimate, you have to estimate the duration. This is closely related to constructing a draft schedule, and inherently involves decisions on how many people you will put on the project. Headcount can, to a certain extent, be traded for schedule, but remember that there is still a minimum duration for some tasks, e.g. it is impossible to make a baby in one month by putting nine women to work on the task.
If you have 2 teleporters that have to be set up, and only one teleportation expert, you are not going to be able to do those three tasks in parallel. Thus the duration will be longer but the effort will remain the same.
Installation and set up of 2 teleporters is estimated to take 80 hours.
If you have two teleportation experts committed to 40 hours per week, the duration would be 5 calendar days.
If there is one teleportation expert committed to 40 hours per week, the duration would be 12 calendar days.
If there is one teleportation expert committed to 20 hours per week, the duration would be 26 calendar days.
These two terms relate strongly to schedule, which is the project timeline. This involves identifying the dates (absolute or relative to a start date) that project tasks will be started and completed, resources will be required and upon which milestones will be reached.
When you are working towards a deadline, understanding and tracking the difference between duration and effort will allow you to schedule the time to spend on other tasks and still make your deadline.
A few classical quotes on this topic from Frederick Brooks book The Mythical Man-Month: Essays on Software Engineering
Men and months are interchangeable commodities only when a task can be partitioned among many workers with no communication among them. This is true of reaping wheat or picking cotton; it is not even approximately true of systems programming. The added burden of communication is made up of two parts: training and intercommunication.
Each worker must be trained in the technology, the goals of the effort, the overall strategy, and the plan of work. This training cannot be partitioned, so this part of the work varies linearly with the number of workers. Since software construction is inherently a systems effort - an exercise in complex interrelationships - communication effort is great, and it quickly dominates the decrease in individual task time brought about by partitioning. Adding more men lengthens, not shortens, the schedule.
I hope this post has clarified how to use the two terms properly.