Military Retirement Conversion to Federal Service

In a few short years I will retire from the Marine Corps after 25 years of faithful service. I will have 20 years active and 5 years reserve time. As I look forward into the future I think about the various jobs available. I see that Federal Service offers a lot jobs for military retirees. What is the conversion from military service to federal service?  For example 25 years of military service gives men how many years of federal service for retirement?