It is important to note that the activities required for a successful project cannot be put off without the whole project being delayed. Usually, these are high-priority activities that need to be finished in order to finish the project within time and budget. Some examples of critical path tasks could be designing a product or getting regulatory approval.
This depends on many factors including the scope of a specific project, its timeline, available resources, and budget. But in general it can be determined by taking into account both estimated time required for each activity along the critical path, plus additional time to accommodate contingencies such as delays. These elements can then be identified to provide an estimation of when the project will be finished.