From what I’ve seen there’s a few things that happen. When making a timeline, if the task seems similar to previous tasks, they will use the previous task’s timeline as an indicator of how long it will take. There is always opportunities for extensions depending on importance, and finishing earlier than previously thought is never an issue. Entire teams, lead by the project manager, will usually have weekly planning meetings. Any subdivisions of the team will have their own meetings concerning due dates and when they are going to finish portions of the project. I don’t believe that remaking the plan because of an early finish is an issue that arises.