Dive into the rabbit-hole on Wikipedia.
This law states that work expands to fill the time available for its completion. Meaning that the more time available for some task, the longer it’ll take to complete. I’m not a big fan of this law as I don’t think it’s true for everything and that it’s too often generalized. Doing the same task for 10 hours, 1 hour, and 10 minutes won’t lead to the same result. However, when your work is interrupted by breaks or postponed, I see some truth in this law. For example, consider having to write an essay for university. Assume it’ll take you 20 hours to do the research and writing. Regardless of whether you’ve got 2, 4, or 8 weeks to hand in your essay, the necessary work and used time will likely remain identical. However, the time between receiving the task and submitting your essay will probably increase. This has some limitations as well, e.g., where breaks are necessary for getting feedback and refactoring your work. If certain actions have an inevitable delay, e.g. always having to wait 2-3 days for feedback from your professor, cutting time will eventually negatively impact the quality of your essay. For me personally, Parkinson’s Law simply suggests that there is a sweet spot for how much time to plan for a given task. Try writing a scientific paper in a day and you’ll likely produce garbage. Plan a month for writing the first page of your book and you’ll likely overthink, scrap, and procrastinate, being less productive compared to just sitting down for an hour. So instead of blindly cutting time for a task, I think you should rather spend a minute thinking about how much time you’d actually need and plan accordingly.