Personally, I do not believe it should be too difficult to design a time line. Of course there will be challenges, but for the most part there should be some sort of precedent where a team can look and devise an accurate time line. Also, a good rule of time is to add time on a task that might be more difficult because it is always better to deliver something early than late. To address the question on updating the whole planned process due to finishing a tasks too early. There should not be so much extra time added where it becomes an issue to finish tasks earlier.