According to the Military Postal Service Agency, military shipments arrive to a "gateway" location stateside (New York or San Francisco). At times, shipments can take 7 to 10 days to reach the gateway.
Upon arrival, shipments are sorted, bagged, and tagged to arrive at a military postal unit overseas. While commercial aircraft are very consistent, there could be a large variation in transit times due to military aircraft schedules, weather, transportation and military operations in theater, or movement of a service member's unit.