I can't speak for Krav Maga -- but there's no real consistent answer to your question across martial arts. Some have only a couple of belts, and you progress slowly, taking a year or more per belt. Others also only have a few belts -- but you get to black belt quickly, and that's when your learning really begins. And some have belts and testing every month... and about a zillion belts, too. Personally, the style I train in only has 4 belts. I moved quickly through the first 3... and I was a brown belt for several years.
Rather than worry about how fast you're getting belts, consider whether you're comfortable with the way your skill is progressing. Are you learning material you're confident you can use, that suits your needs? If progression controls which class you can attend, like in some schools, do you feel that you're moving towards the next class at a reasonable rate? Expect to plateau sometimes, and even regress, but if you feel that, overall, you're moving forward, you're probably fine.
