Under the modern U.S. Naval ROTC system, graduates become active duty officers, rather than reserve officers, and are required to serve a term of 5 years for the Navy Option and 4 years for the Marine and Nurse Options. The Naval Reserve Officer Training Corps commissions individuals into either the United States Navy as an Ensign or the United States Marine Corps as a Second Lieutenant. While attending college, these prospective officer candidates are known as Midshipmen. Whereas Naval Academy Midshipmen are on active duty, NROTC Midshipmen are in the Navy Reserve but are on active duty for periods of training during the summer. The primary difference is that NROTC Midshipmen attend an ordinary civilian college or university, whereas Naval Academy Midshipmen attend the U.S. Naval Academy in Annapolis, MD, which is a much more regimented, military environment.
Scholarship Midshipmen are those who applied to the Navy for an ROTC scholarship (during their senior year of high school, or during early college studies). Some NROTC students have served as enlisted men or women in the Navy or Marine Corps. The highly selective application process involves an extensive written application and an interview with a Navy representative. Applicants must also pass an entrance medical examination process. The Navy pays tuition for Scholarship Midshipmen, educational fees (i.e. lab fees), as well as a stipend for books. All Midshipmen fall under one of three types: Navy Option, Navy Nurse Option, or Marine Option. The Navy does not pay for room and board; however, some schools will offer scholarships to cover at least a portion of room and board. In addition to tuition, the Navy pays a monthly stipend during the school year. As of 2011, the stipend was $250 per month for first-year Midshipmen, with a $50 increase each year after that (i.e. $300/month for sophomores, and so on).
College Program Midshipmen are those who join Naval ROTC without a scholarship. They complete all activities and requirements of scholarship midshipmen and if they continue in the program for four years will also be commissioned as Ensigns or Second Lieutenants. They will often be offered a scholarship by the Navy if they perform well academically and within the ROTC program. Because of the technical nature of the Navy, students entering college without a 4-year scholarship who are planning to major in a technical field, such as engineering, science, or math, are more competitive for the scholarships.
Enlisted Marines participate in ROTC through the Marine Enlisted Commissioning Education Program (MECEP) and are referred to by their rank or by the name of the program, "MECEP." (pronounced mee-sep) MECEP Marines do not have their school paid for by the Marine Corps and generally use the Montgomery GI Bill or the Post-9/11 GI Bill to pay for school. They continue to receive pay in accordance with their rank, however any promotions while they are attending school are considered non-competitive and will be revoked if they fail out of the commissioning program.
The normal, "baseline" service commitment for Scholarship NROTC graduates is eight years, with no less than five served on active duty. The exact commitment will depend on which warfare community a Midshipman is selected for. For example, Navy and Marine pilots are generally committed to eight years after their date of winging. Because the training for a Naval Aviator is extensive, this can lead to a commitment of up to 10 years. Naval Flight Officers usually serve a six-year commitment and Submarine Officers usually serve a five-year commitment.
Commitment is also based on whether a midshipman is enrolled in the scholarship or college program. Those midshipmen who are in the college program typically only owe three years of active duty service.
Once a naval officer completes their active duty commitment, they must serve the rest of their three years in some portion of the Navy Reserve.
I would suspect in the situation you mention the person could get away with an arrangement for say a year of active service and 3 years of reserve, of maybe something like 5 years of reserve only. You really need to speak to a service recruiting people for a definitive answer - or you can fudge it the way I did.
I had my guy attend most, but not all, of the courses and he was a member of the state militia where he came from, so they counted that in his favour to approve him doing the ROTC classes.