Speaking as someone who has a black belt, I think that belt ranks are generally the last thing you should worry about. Sure, you can be proud of having earned a black belt, but that shouldn't your main goal. If you concentrate on polishing your technique and conditioning yourself, a good teacher will let you know when you're ready for the next rank.
That does assume of course that you have a good teacher. If you're worried that you've joined a McDojo, or that your instructor is "prostituting" his art, you can always check out another school or gym to see what their standards are like.
I'm told that the record for someone earning their black belt in the World TKD Association was 1 year and 9 months, but that that was a very exceptional case. This guy apparently had a military backround, years of prior martial arts experience in other styles, and his hobbies included every sport he ever heard of. Besides that, since he had to test for his black belt with the grandmaster (just like everyone else), I don't doubt that he truly earned it. (Though I would understand the skepticism of anyone who never met the grandmaster.)