Project Estimated Effort is an allocation of planned working hours between different categories of project activities. Estimating of an effort means planning the amount of time (effort hours) that you need to accomplish all the project tasks and to produce all the project deliverables you have in the list, so as a part of deriving your project estimated effort you need to have at your hands an accurate project breakdown plan (your work decomposition).
While you can calculate your project estimated effort as, let’s say, 500 effort hours (meaning that it is a total net amount of hours you need to do the work, computed as a sum of estimated durations of all particular tasks involved into the project, including additional time allocated for administrative issues, etc) the factual project duration will be shorter or longer – this depends on how you are going to allocate these estimated hours within days of the project schedule (for example you can complete whether 5, 10, 20, 30 or more effort hours daily).
Amount of effort hours you can complete daily depends on resources engaged (their amount and working capacity), and delays which can be anticipated. As the project progresses and the scope definition improves the project managers are able to better accurateness of the effort estimates.