Enlistments generally run 4 years, although there are lots of exceptions. The services are free to negotiate enlistment contracts pretty much as they wish, so often the length of enlistment may go up in exchange for other considerations. For example, some technical training schools can be guaranteed if you sign up for 6 years. I don't recall ever running into an enlistment of more than 6 years or less than 3 years. Generally an enlistment can be extended by up to 2 years by simply filling out some paperwork. This is usually done for the convenience of the government (very rarely for the convenience of the individual). An example might where an individual is going to an overseas assignment that would normally last 4 years, but only has 2 years left on his enlistment, he would be offered the assignment and a 2-year extension. This pops up when looking at named and dated Navy and Marine Corps Good Conduct Medals, where you'll often see odd terms of service caused by enlistment extensions. There are even bars on Good Conduct Medals that say "Extension of Enlistment". The total length of time one can serve is a combination of age and rank, but service over 30 years takes special approval and is usually only offered to the highest ranking NCOs, and then usually only if they are in positions of substantial responsibility. There are hundreds of pages of policies and manuals on this subject and these change constantly as the services try to manage their forces in the long term. Also, citizenship is not a requirement for enlistment, but may be required to hold certain positions or security clearances, which will limit the jobs available. I once had a situation where one of my NCOs had very high level security clearances, which required citizenship and serious background investigations. When he was selected for an overseas assignment that required an official passport, it was discovered that he was a citizen of Panama, not the US. Much huffing and puffing followed about punitive actions against the NCO, until it was discovered that he had always noted his Panamanian citizenship in his records and the security clearance guys had fouled up in giving him access. Some strings were finally pulled to get his US citizenship processed and approved.