20/80 Dev Time Rule
The final 20% of development takes as much as 80% of the total development time.
The last 20% of development takes 80% of your time! This may sound like kind of a silly one, because if it takes 80% of your time, wouldn’t it be 80% of development? The point of the 20/80 (or 80/20) rule is that most devs severly underestimate how long it takes to polish a game. They will budget around 20% of their timeline for polish, when really it takes around 80% of your total time. That miscalculation means a dev timeline estimate will be 60% shorter than it should be, simply due to this underestimation of polish time.
Of course, there are many additional factors that influence development time, and in general, I find that devs underestimate time to complete a commercial release by a factor of 3! Seriously.
The Witcher 3: The Wild Hunt