Bottom line
There are two ways to come into the enlisted Reserve. You either join with no prior Marine Corps time (Non-Prior Service, or NPS) or you come in after finishing your active-duty or initial drilling obligation (Prior Service, or PS). About two-thirds of enlisted accessions are NPS. NPS Marines enlisting into the SMCR sign an eight-year enlistment. Either way, your unit owes you an orientation brief shortly after you join.
What it means for you
If you are coming straight off the street into the SMCR, you sign for eight years and your unit should welcome you with an orientation brief, normally within 30 days of enlistment. If you already served and you are affiliating with a Reserve unit, prior service recruiters handle your screening, and you still get an orientation brief, normally within the first 30 days of affiliating. The orientation brief is a welcome and a get-acquainted, not a screening to decide whether you belong. That screening is the recruiting command's job.