Gradings are typically held every three months in March, June, September and December. Students are required to have regularly attended classes throughout the period and demonstrated an improvement in standard. The final decision on grading rests with the club Chief Instructors.  A typical grading will cover the three "Ks" - Kihon, Kata and Kumite. These increase in complexity as you progress through the ranks.
Pre Gradings - a pre grading will be held in the weeks leading up to a formal grading which will be be held similar to grading conditions - covering all the requirements of the relevant grading. Feedback will be provided along with a final decision to grade or not. It is the responsibility of the student to know the requirements of their next grading, so study this in the Syllabus section.
There is never a need to "chase grades" and race throught the belts, as this devalues the grade held. A grade should always reflect the standard of the student - always better to be a standard above the belt you are wearing than be below it!