Bottom line
Routine dental care from the Marine Corps kicks in only when you are on Active Duty (AD) for more than 30 days. Short of that, you are not entitled to routine care, but you can still get coverage through the TRICARE Dental Program (TDP) for yourself and your family. If you are in the SELRES, you also get a yearly oral exam to keep you mobilization-ready.
What it means for you
The 30-day mark is the line that matters. Orders for 30 days or less do not entitle you to routine dental care, and they do not unlock the active-duty dental entitlement for your dependents. Cross 30 days on a single tour and you get full dental coverage, and your dependents become eligible for the same active-duty-style dental entitlements through TDP. The rest of the time, your dental safety net is the TDP, which you sign up for and pay into. SELRES Marines get one type 2 oral exam a year on top of all this, so a dental problem does not catch you off guard if you mobilize.