On my interview day, I believe I was told that the term of military service is seven years of active duty.
I don't know what your situation is...whether you have been accepted at USUHS...other medical schools...etc...but I would definitely make sure you know what you are getting yourself into if you decide to go to USUHS. Military medicine takes a lot of committment and is definitely different from what most people expect of civilian physicians. After my interview, I realized USUHS was not the right place of me. Fortunately, I was accepted to a few other medical schools so I had no hesitation about withdrawing my application.
Good luck to you whatever your situation may be.