The project manager is the person who must create a schedule and believe in it. In many cases the project manager has the expertise to create the schedule from scratch. However, as projects become more and more complex, he may not have the expertise to build the plan entirely on his own. There are a number of techniques you can use when you don't know all of the information required to build the schedule.
Use a pre-existing workplan
The project manager may not have managed a similar schedule before, but someone else in the organization may have. If your organization saves prior project schedules, you may be able to find one from a similar project. This can go a long way toward helping you create a realistic schedule for your project.
Use a schedule template
Your organization may not save prior schedules, but you may have access to schedule templates. For instance, you may have schedule templates for iterative development, package implementation, researcher projects, etc. These templates may provide 80% of the activities that you will need on your project and can be utilized with a little customization.
Create a draft and circulate to stakeholders.
In this approach, the project manager creates an initial draft of the schedule as best as he can. There may be a number of holes identified that the project manager does not feel comfortable to fill in. When the draft is completed, it's circulated to the project team members and other stakeholders for feedback. These stakeholders may be able to fill in the missing pieces and validate that the final workplan is sound. During the review process, work is added, changed, or deleted. The project manager takes the feedback and incorporates it into the schedule, which is then used going forward in the project. This approach results in a very good schedule and provides opportunities for feedback and buy-in from the stakeholders.
Build the WBS and schedule through direct stakeholder involvement
In this approach, the schedule is actually built through one or more sessions with the project team members and other stakeholders in the room. Each person may have a specific way of viewing the project, but a complete schedule can be generated by gaining a consensus with all of the people in the room. This approach also has the advantage of having active engagement and participation from the stakeholders.
The project manager needs to take ownership
Regardless of how the schedule is built, the project manager must accept responsibility for the final result. It is not good enough for the project manager to say he's not accountable because he didn't know all the details. If the project manager doesn't know all of the specific work required to complete the project, he must determine the best way to fill in the gaps. He must then take ownership and be responsible for the results.