At least 3 years for Duke.

• "Enroll in the best available and most challenging courses. We recommend four years of English and at least three years of mathematics, natural sciences, foreign language, and social studies. We generally expect students to enroll in five academic courses per year, and if a student does not take four years in a particular subject area, it should be replaced with an academic course of equal rigor. For students applying to the Pratt School of Engineering, we require coursework in calculus and strongly recommend physics. We also encourage students to enroll in advanced-level work in as many areas as reasonable, regardless of your intended major. For some students, this will include AP or IB courses, whereas for others it will include honors, accelerated, or college courses."
• https://admissions.duke.edu/application/overview?