An old tip from a course I took years ago was, Make your best guess and add 50%. There are many factors besides "you" that result in things taking more time than you expect.

That being said, I prefer not to be abrupt with "timewasters" i.e. people, and choose option b, above. In the end, people are more important than tasks, and I don't want to turn people into tasks.

Best Wishes,
Gordon