The Shotokan syllabus is divided up into different levels of technique.
After each Obi (belt) is awarded you will also be given a certificate of achievement with your Obi.
Typical timings to achieve each grading is as follows:
10th kyu to 3rd kyu three months training twice a week.
3rd kyu to 1st kyu six mouths training twice a week.
1st kyu to 1st dan black belt twelve mouths training twice a week.