Parkinson's Law is the adage that "work expands to fill the time available for its completion", suggesting that the duration of a task is dictated by the deadline set for it, rather than by the amount of work involved. This concept implies that if you give yourself a week to complete a two-hour task, then (psychologically speaking) the task will increase in complexity and become more daunting so as to fill that week. It highlights the inefficiencies in time management and suggests the importance of setting appropriate deadlines to increase productivity.
A student given two weeks to prepare a presentation will likely use the entire period, even if the actual work could be completed in a few days.
An employee assigned a minor task with a long deadline might over-complicate the task, making it more time-consuming than necessary.