"The last time I had to hand in an essay" will probably be the most common answer - if you can come up with anything more original it will probably be appreciated! This is an opportunity to give an indication of your ability to plan your work, organize your time and handle several competing priorities - more essential skills for a solicitor.

Consider including in your answer:

Any restrictions or limitations that risked your meeting the deadline

Your effective prioritization of tasks which enabled you to be on time

The need for initial planning and organization

How you handled conflicting demands from other sources

The need to be focused on the task at hand.