In contradiction to Parkinson’s Law

Things usually take longer than you expect, even when you take into account The Hofstadter’s Law itself. ¹

E.g., if you think you’ll finish your homework in an hour, it might actually take two hours.

The Ninety-Ninety Rule in Software Engineering

The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.

The final 5% of the work costs an extra 100%. In other words, it may take 100 units of effort to get to 95% quality, but that final 5% will take another 100.

When you have 90 percent of a large project completed, finishing up the final details will take another 90 percent.

Often, the last 10% of a task takes as much time as the first 90% did.

See also: The Goal-Gradient Effect