How long are most military helicopters used by the military?

The U.S. Marine Corps and the Navy are currently operating some helicopters that go as far back as the Vietnam War. The do however go to be reworked. The CH-46 came out in 1964, it has been upgraded several times throughout it's life time. I believe the CH-46 are some of the oldest airframes still operating in the U.S. military. The CH-53 came out in 74' but new airframes were continued until as late as 1997. From there I'm not sure. The Marine Corp also operates some Specialty VH-3D's for Presidential Flight Duty whose airframes saw duty 35 years ago.

Hope this gives a little insight. These aircraft are kept in service a long time, but the maintenance is outstanding keeping them in tip top shape for years and years.