Normally you do 4 years at a time and then if you want to do 4 more you put in a packet and they look back at the 4 years you did and decide if you can do 4 more. Sometimes you can do 6 years at a time. Good luck! The marine corps is not always an easy lifestyle but I would not change it for the world!
A normal tour of duty in the Marine Corps is 4 years. A Marine Corp can request for another 4 year tour of duty one he completes the first 4 years of normal tour of duty. For this second tour of duty, your previous tour record will be scrutinized to see whether or not you are capable and fit to qualify for another tour of duty.