Unlike most other martial arts, belts are not universally adopted in Krav Maga. In fact, in many schools’ belts are not even used! Instead of belts many schools use grades or groups named Practitioner, Graduate, Expert, and Master. Patches are used by some, belts by others, and nothing at all by some schools.
The school I attend is affiliated with Krav Maga Worldwide (which itself is based on a 5 belt system). You start out not as a White belt but as a Yellow belt (equivalent to Practitioner), progress to Orange (Practitioner), Green (Practitioner/Graduate), Blue (Graduate), Brown (Graduate), then through 5 degrees of Black (Expert/Master) — if you live that long.
As I understand it, as my school became affiliated with Worldwide, there was some negotiating with the organization to keep the belt system already in place, a system that the school founder and teaches believed very strongly in. This was agreed to and today the school has 10 belts: White, Yellow, Orange, Purple, Blue, Green, Brown, Red, Red/Black, and Black. The belts take various amounts of time to achieve depending both of how often you train as well as the belts themselves (i.e., some belts just take longer, such as Black).
Further, my school has two broad classes for adults, a Basic Training group (Practitioners, I presume) as well as an Advanced Training group (Graduates and Experts, though I am not sure where the line is in the belt rankings). The Basic group contains us newbies and ranges from White belt up through and including Orange belts. Most of the group is out of shape, sheepishly watching the Advanced students from the sidelines as they confidently strut their advanced stuff and pretend they’re not being watched.
As I understand it (again, at least in my school), it’s customary for a student to be part of the Basic group for about 9 months. The promotion through each belt in the group includes a stripe on the belt during the last week of each month. If the student is showing the right progress they get a stripe. After three stripes, a belt promotion.
Once one is bumped out of Orange into Purple they start all over at the “bottom” as they get promoted into the Advanced class, a place where, in some ways, the “real” training for Black belt begins.