In this task, you're given passages that contain mentions of the quantity of a particular object and are supposed to find out the answer for a given question. Craft one correct answer to the question given in input. Make sure your correct answer is consistent with the context, and properly gives the exact number relating to the query.
 In your answer, use as few words as possible from the given context. To be less repetitive, please vary your language for each question.

Sentence: In 1919 Father James Burns became president of Notre Dame, and in three years he produced an academic revolution that brought the school up to national standards by adopting the elective system and moving away from the university's traditional scholastic and classical emphasis. By contrast, the Jesuit colleges, bastions of academic conservatism, were reluctant to move to a system of electives. Their graduates were shut out of Harvard Law School for that reason. Notre Dame continued to grow over the years, adding more colleges, programs, and sports teams. By 1921, with the addition of the College of Commerce, Notre Dame had grown from a small college to a university with five colleges and a professional law school. The university continued to expand and add new residence halls and buildings with each subsequent president. 
Question: Over how many years did the change to national standards undertaken at Notre Dame in the early 20th century take place?
three years