I've been told that stripes are supposed to represent a certain amount of hours on the mats (100+ hours or something like that). However, I don't really think that's the only factor that determines when you earn a new stripe or belt. I am not a black belt so I am not entirely sure what goes into making that decision to promote someone. But I've been told it is based on progress and dedication as well as attitude, contributions to your academy, helping teammates, self defense knowledge, being a good person, being respectful and tournament success. Each school has a different criteria for earning a promotion.
Some schools are more regimented than others and have specific areas of knowledge that must be gained prior to promotions. Whereas other schools are a bit more easy going and they don't have a set formula to determine when a promotion is in order. Each instructor is different too.
In the Gracie's system (Relson Gracie in my case) you receive four stripes before you are promoted to the next belt level from blue belt to brown. Once you are a black belt, the rules get a little different. A black belt can have up to 7 or 8 stripes before changing out to the highest of honors that one can achieve, the 9th degree. Although there is a higher rank than a 9th degree, the 10th degree is permanently reserved to the pioneers of Brazilian Jiu-Jitsu: Carlos, Oswaldo, George, Gastão and Hélio Gracie.
Recently a white belt asked me how long it took me to get my blue belt and how much I trained before I did. The truth is there is no magical amount of time that it takes to advance in Jiu Jitsu. Everyone learns differently and I believe promotions are not something to go chasing after. The point is to learn, have fun and grow. Just be the best you can on the mats and don't worry about what color or how many stripes your belt has. It's what's on the inside that really counts.