The Brazilian Jiu-Jitsu Belt System is a far stricter than in most other martial arts. It can take upwards of 10 years to achieve the black belt.
As there are only a few belts, you will spend a long time on each one. Also, your progress will not be linear. You will spend weeks or months on plateaus before being rewarded with a jump in ability. Sometimes it will actually seem as if you are getting worse instead of better. But the plateau is actually your friend and is itself an indicator of progress. It is those who have the mental strength to keep going when they find themselves on a plateau that break through to a higher level of ability.
At each belt you will earn 4 stripes before promotion to the next color rank. It takes an average of 3 months to earn a strip in the youth ranks and can take 4-6 months to earn a stripe in the adult ranks. Black Belts earn rank stripes every 3+ years. A black belt with white stripes on each side of the red rank bar denotes "Professor" status. An adult color rank with white stripes on each end of the black rank bar denotes an instructor.