Estimating duration in a project is a daunting task that is usually a best guestimate based on past history. If everyone worked eight hours per day, which is usual in the U.S., but not in India (usually a 9 hour day), and they were 100% productive for all eight hours, you could calculate duration by taking the number of effort hours, divided by the number of resources. So, if one person is assigned a task activity that is estimated at 80 hours, and she works eight hours per day, the duration would be (80 hours / 8 hours per day) = 10 days. Similarly, if four people are assigned to the same task activity full time, the duration would be divided by 4 giving you 2.5 days (10 days/4 = 2.5 days).
However, no one really works a perfect 8 or 9 hours. Our work day is generally broken up into pieces in which other activities such as answering emails, lunch, meetings, other assigned activities, etc. A better estimate would use the 80-20 rule in which 20% of a resources time is eaten up by activities not related to the assigned work. Therefore, I would suggest using the following process to determine duration:
1. Estimate the productive hours per day
A rule of thumb I was taught is using a factor of 6.5 productive hours per day helps you take into consideration those other activities such as answering emails, lunch, meetings, other assigned activities, etc. Using the above example we find our answer for an activity estimated at 80 hours / 6.5 hours = 13 days for one resource, while four resources would take 3 days.
2. Determine the number of resources needed for each activity
Knowing that the more resources you apply to activities, the earlier you can complete the activities, obviously two resources may be able to finish an activity faster than one person, but it may not necessarily be twice as fast. At some point, additional resources will not make the activity finish any sooner, and could possibly, make it go longer.
3. Determine available workdays
You need to take into account holidays, vacations and training, especially when using international resources. This was not included in the example from number one above, since this non-project time can be scheduled and accounted for in advance. On a twelve-month project, team members will be out for vacation days, holidays (US and international) will need to be accounted for in your schedule. To make your schedule more accurate, take into account any days that you know your team will not be available to work on the project.
4. Determine resource time allocation
Account for any resources that are not full time. Keep in mind that a resource whose allocated only 50% of his time to your project will take twice as long to do any individual activity. Using our previous example, you have an activity that has an estimated effort of 80 hours, and you assign a resource that is only allocated 50% to your project, the resulting duration will be at least 25 days for one resource, if not more. For four resources our duration would be 7 days.
5. Calculate delays and lag-times
Some activities have a small number of effort hours, but a long duration. For instance, a deliverable approval may take one hour, but might take two weeks to schedule the meeting.
6. Determine constraints
When building your schedule, identify the tasks that need to be done sequentially and those that can be done in parallel. If you have enough resources, all of the parallel activities can be done in parallel, but only if you have the right resources available at the right time. You may have activities that can be be done in parallel, but you have only one resource to them, thus they have to be done sequentially.
7. Document assumptions
Most importantly, you will never know all the details of a project. As such, it is important to document all the assumptions you are making along with the estimate. The more you communicate with your stakeholders on how you arrived at your conclusions, the better able they will understand your plan and be more willing to accept it.
Using the above suggestions should help make estimating durations for your projects much easier.