How true is Parkinson's Law?

Parkinson's Law: Work expands to fill the time available for its completion.

Which basically means that if you plan 2 hours for a project, then the project will take 2 hours, regardless if others can do it in half the time. Something will make the project last as long as you schedule it.

If your into Productivity, you should read more about Parkinson Law and the famous Cobb-Webb theorem.