There is a project management rule, semi-serious, that says to get the real length of time for a project you should double the estimate and increment the unit. So if the estimate is two days, then it's actually going to take four weeks. If estimate is one week, actually two months. What is this rule called and what is its source/origin?
This rule is commonly known as Parkinson's Law, named after Cyril Northcote Parkinson, a British naval historian and author, who first described it in a 1955 essay for The Economist. The basic idea behind Parkinson's Law is that "work expands to fill the time available for its completion." In other words, the longer you have to complete a task, the more time it will take you to finish it.
No, that's not it. It is very specifically a rule about how to get the real length for a project, and it involves incrementing the time unit, from days to weeks, or from months to years, etc.
I apologize, you are correct that the rule I described earlier is not the one you were thinking of. The rule you described is often referred to as "Parkinson's Law of Triviality" or "Bike Shedding" which suggest that the time spent on discussing and deciding a trivial issue will be much larger than it should be, while a important issue will receive less attention than it deserves. This rule is often used to describe the phenomenon of a group discussion or meeting that starts with an important topic, but quickly devolves into a discussion of a trivial or insignificant matter.
It is not a rule that help you to estimate the real length for a project, but rather a rule that describe a common behavior in the decision making process.