Effort and Scheduling (alternatively: Effort and Duration) are not the same, as every Project Manager knows (or should know). It is startling how many people confuse the two, leading to all kinds of mayhem.
Here Mr Frog has asked a question about effort, and received an answer, but he then makes the canonical error of thinking it is easy to map effort onto a calendar to derive a scheduling.
If you’re asking about how much work something is, that is Effort. The unit for human effort is person-days, or some variant thereof (person-weeks, person-months etc etc), and its calculation is trivial multiplication if you know both inputs:
|# of people||length of time for each||total effort|
|2||1 days||2 person-days|
|1||2 days||2 person-days|
|2||1 week||2 person-weeks|
|5||2 months||10 person-months|
If you are asking about calendar dates (often, when something will start, or – more importantly – when it will finish), the topic is Scheduling, and the unit is calendar days (or weeks, months etc). The below shows 2 calendar weeks, not to be confused with 2 person-weeks as above; you might be able to do 2 person-weeks of work in an hour, if you have enough people. 2 calendar weeks takes 2 weeks, by definition.
There is of course some relationship between Effort and Scheduling for any given piece of work, but it is not a simple one, depending on such things as resource availability, possibilities for parallelisation, and the massive uncertainties present in all complex work.
It doesn’t help matters when people confuse the inputs and outputs of such calculations, which is why it is imperative to keep these two concepts separate, and to be cautious and show your working when trying to derive one from the other.