Ninety–ninety rule

The ninety-ninety rule is a saying that basically means that the first 90% of a project will take 90% of the time and the last 10% will take the other 90% of the time. In other words, it will take a lot longer to finish the last 10% than the first 90%. This is because the last 10% usually contains the most difficult problems to solve, and they can take a while to figure out.