The regulation allows for 5 1-year extensions out to a maximum of 6 years; however, each service may adapt those rules as they see fit. When I retired from the USAF in '14 they were permitting up to the full 6 years of extensions.

My last mil move was a definite kick in the balls to help remind me why I was leaving. A simple 1000 mile drive with no intermediate stops or storage drop-offs and more stuff broken than any move before. I had furniture that had successfully crossed the Atlantic 3 times which met its final demise between San Antonio and Fayetteville.