Kendo grades work on a Kyu and Dan system much like other martial arts.
These start at seventh kyu and progress to first kyu (Ikkyu). The kyu grades are equivalent to the coloured belt awarded in other martial arts. Ikkyu is the first grade the requires national testing.
These grades goes from 1st Dan (Shodan) to 8th Dan (Hachidan). These are the adult grades and will take a lifetime to progress though. The minimum time anyone can take to go from Shodan to Hachidan is 36 years. However this would require not failing a single grade! For perspective, the pass rate for hachidan exams is commonly less than 0.1%
More Infomation on Grade requirements can be found on the BKA website here.