There is no specific average, but the most commonly seen lengths of enlistments in the United States Marine Corp are 3, 4 or 5 years of service. Someone that is looking to retire from the Marines will serve anywhere between 20 and 30 years at the max. Most people who sign up with the United States Marine Corp enter immediately into boot camp and begin their training, but there are other options if an enrolling person doesn't meet all of the requirements.
The delayed entry program allows service men and women to enlist, but may not be sent to boot camp or begin training for up to a year. This helps ensure the young men and women that are joining are in the best mental and physical shape they can be prior to starting their training.