When you enter the armed forces of the United States you inccur an intial service obligation of no less than 6 years and no more than 8 years. This service obligation can be met by spending 4 years on active duty and 4 in the IRR or any combination. It can also be met by serving 8 years on active duty. Re-enlistments do mitigate time spent in the IRR. After 8 years of service regardless of how it is broken down service members have completed their intial service obligation.
Refrences US Code, Title 10 (DOD) Chapter 31 & Chapter 1005.
Here's a great link to Title 10 of the US Code if you want to check:
http://uscode.house.gov/title_10.htm
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Service Members activated in the IRR can only be kept on active duty for a period not to exceed 24 months.
Service Members extended on active duty "Stop-Loss" can be kept on active duty until the war or national emergency is declared over + 6 months.
For the IRR to be activated the Secretary of Defense must appoint a council to determine who to recall the have to consider time all ready spent in service, family obligations, personal hardship. Each postion that is to be filled by an IRR perssonnel must be considered by the council seperately. ie rank, mos.