That depends on whether you want to be a licensed interior designer. The requirements vary by state. Anyone can call themselves an interior decorator or a designer but some states like Illinois have title laws. In these states you cannot call yourself an interior designer unless you have passed a licensing exam. You qualify to sit for the exam with a total of 6 years of education and experience, meaning a 2-yr Assoc. degree plus 4 yrs of full time work experience or a 4-yr Bachelor's degree plus 2 yrs of full time work experience. See NCIDQ (National Council for Interior Design Qualification) online for more info. There are also practice laws that vary by state.
There are many different paths to seek education in Interior Design. Many community colleges offer interior design certifications in specialty areas of design like Kitchen & Bath, Rendering, Green Design, etc. These take less hours to accomplish than an Assoc. degree and are a good option if you need to get into the field quickly to make income. The hours earned in these certificate programs can also be used to earn an Assoc. degree. To work in commercial interior design, you almost always need a bachelor's degree from an accredited school. There are enough designers that have that degree to make jobs competitive. Not many individuals seek a master's degree in interior design. Those that do usually want to do research and/or teach.