Yes Jules. I know a SSgt that was busted to Sgt and a couple years later had to get out because he had hit is 15 year mark or some such. Same goes for officers. I just dont know the time frames for all the ranks.
Used to Ssgt in 12 but you could get a waiver depending on the availabity of Staff billets in your MOS. Miss it during your waiver period and your out.