From what i have seen and been told by the recruiters your minimum time is 8 years, 4 years active and 4 years inactive service. during this time of innactive service you are able to resume a civillian life, but the marines reserve the right to call you back at any time during this period.
some special jobs require you spend 5-6 years active, jobs with alot of training involved, or require alot of money to be spent on your training usually.
I hope this answer helps you or anyone else who is looking out