Answer : It depends on the program. How long it is for an interior design degree ranges from two years to seven. Interior design is just one aspect of the profession, there are several different types of degree programs available like landscape architecture. The career has many opportunities and so it might be good to do your research to find out which type will work best for you before picking a college or university and spending time studying for something that you don't want to do in the end.
More information about what a career in interior design entails can be found here and more information about education requirements can be found here which shows that some colleges may offer either three year undergraduate degrees in landscape architect or four year graduate degrees.