Move The Essay Back, Please!

Riddle: Sofia is a student in high school. Her professor, Ms. Davis, assigned her an essay that would be due in three days. Sofia, being the worst student in her class, procrastinates and doesn't even begin the essay. Three days later, Sofia begs Ms. Davis to push the essay date back by five more days, and Ms. Davis agrees. Five days go by, and still…no essay. Sofia begs Ms. Davis to push the essay date back by a week, and Ms. Davis agrees. One week later, Sofia comes to the class with a burning candle (How did she even bring it in there?), but no essay. Sofia asks Ms. Davis if she can push the essay back until the candle wick burns out, and Ms. Davis agrees once again. Sofia laughs because she doesn't have to ever worry about the essay again. Why?