What is the method for estimating the timeline for the critical path of a project?

My boss asked me to come up with an independent estimate of the timeline for a large project that involves several departments and outside contractors.

Defining the critical path of a project involves identifying all of the steps required to complete the project, determining how much time each step will take, and determining which steps are dependent on other steps (some steps require certain other steps to be completed before they are started while other steps may be completed at any time). Once you have gathered this information, the critical path is the longest sequence of steps required to complete the project. While the concept of critical path is useful in estimating project duration, you also need to make a practical assessment of when people actually plan to complete their project assignments - while the critical path analysis may suggest a certain timeline for the project, you may find that plans or schedules unrelated to your specific project can extend that timeline significantly.