I found an interesting law, rule or whatever you choose to call it, when I read about software development (Eng. Software Engineering). Parkinson's Law states ( Wikipedia ):

"Work Expands so as to fill the time available for ITS completion."

Parkinson's Law means that if you put a time on a project or work so the project will be as long or longer. Sure, it may take less time, but usually is the complexity of the project so that it continues until the time (or budget) are exhausted.

Conducts a project

The law is the textbook author (Sommerville, 2007) as applied to IT projects, but in my opinion it should also be applicable to the type of project at any time. Especially if the project is planned or follow any particular process. See my discussion of how management consultants are working .

References

Sommerville, Ian (2007). Software Engineering. Boston: Addison-Wesley