C. Northcote Parkinson stated in this article from a 1955 edition of the Economist (PDF):
“Work expands so as to fill the time available for its completion.”
This is regarded as “Parkinson’s Law.” In the case of studying for an exam, when more time allowed to by spent on the task of studying, there will not necessarily be create increased results, but there will be simply more time spent on studying. The less time allowed to be spent on studying, the less time will be required for studying. This principle is true for many different types of work – greater time spent on the task will not necessarily yield greater results.