COMPLETION TIME. No matter how long you think it will take to write a paper based on your research, see the paper you just submitted in print, complete a research project, prepare a new course,or prepare for a session of a course you gave previously it will take longer. The wide-eyed optimists always think the task will be completed on time. The mildly realistic optimists think the task will take their estimated time plus 10%. The pessimists understand that the delay is at least 50% on average.
(Corollary: Even if you add the above delay times to your estimate, it will still take longer than that.)