One of the most important things that you will do as a project manager is to estimate how long your project will take, and how much work will be required to complete the project. These two concepts are closely related , but most project managers use either one or the other when making estimates of their projects.
Many project managers go through their list of tasks and estimate how many working days will be required to complete the task. Then when resources are assigned to the tasks , the number of hours of work becomes clear, depending on how many resources are assigned and their available working hours. Other project managers go about this from the other angle: They estimate the number of hours of work that will be required to complete each task, and then let the duration be determined by the number of resources assigned to the task.
No matter which method you use, it is important to remember that almost everything else you do as the manager of your project's schedule depends upon the estimates you make for duration or work. If you estimate too little time for a task, your schedule may slip, causing delays or political and career issues for you and your team. If you estimate too much time, the project may be cancelled because it will cost too much or take too long.