This is about aligning expectations and gets to why the project was initiated in the first place. Think about what the final deliverable is and work iteratively backward. Gather stakeholder input and identify reasons the project could end. Setting aside obvious failures, such as lack of funding or losing interest, focus should be on the needs to be delivered and how to conclude the project. Many projects will require ongoing support and maintenance, but this should be clarified up front. The answer to when the project is over shouldn’t be assumed, until asked.