Parkinson's Law is the idea that work expands so as to fill the time available for its completion.
When I first heard about Parkinson's Law I thought "No way. That kind of irrationality applies to other people. Not me." I was wrong.
The most obvious domain Parkinson's Law applies to me is media consumption. If I allow myself to watch YouTube, play videogames and follow the news then media consumption will consume all of my free time. I solve this problem by putting limits on my media intake.
Another domain Parkinson's Law applies to me is direct asynchronous electronic communication. I check email all the time unless I create a schedule. The same goes for reading blog comments.
If I allow myself a week to prepare for a dance then it will take me a week to prepare for the dance. If I allow myself one day to prepare for the same dance then it will take me one day to prepare for the dance.
The amount of time I allow myself is often anti-correlated to the quality of my output. My writing (especially fiction) tends to be better the faster I write it because tight time constraints force me to include only the most important words.