How to confirm the duration

Eddie,

Since each project (EPC)will be unique, historical information may be of little use in this case. Therefore individual department personnel (Engineering, procurement, Construction)people should come up with estimated durations for the activities based on the nature & complexity of the project which in turn depends on the experience of individuals involved in the project.

Regards
Naveen

Eddie,

At early stage, we can use benchmarking, meaning, if the project is of similar nature that was done in the past .... and it took 2 years to complete, then, the probalbe duration of the same type or nature of project maybe two years also. Sometimes, the client may opt for shorter duration since lesson learn or new technology or advance construction methodology may have been discover

You can also use statistical data, theory of probability, standard deviation, etc., etc.