I read on the Army site that it was 8 years, regardless of active or reserve.
However, I have read elsewhere that it could be six if you do active.
And I have no idea about national guard.
Call 1-888-550-ARMY if you would like to confirm the exact details of your path.
Typically "Scholarship winners must serve for four years; non-scholarship Cadets who enroll in the ROTC Advanced Course must serve for three years. All who graduate and complete ROTC training are commissioned as Second Lieutenants in the U.S. Army.", followed by a reserve commitment. Keep in mind that a reserve commitment *can be mostly active duty deployment, depending on the needs of the service.