Answer : Assuming you mean "How long did it take you to become a specialist in the field of interior design?", the answer is 7 years.
I started out as a general designer, working on small projects here and there. After a few years, I decided to focus exclusively on interior design. I took some classes, worked with a mentor for awhile, and eventually got my degree in interior design. Following that, I spent a couple of years working in the industry before finally hanging up my own shingle and starting my own interior design business. So all told, it took me about seven years to become a specialist in interior design.