Bottom line
You are subject to the UCMJ only while you are in a qualifying duty status. That means while you are on active duty (AD) or active duty for training (ADT), or while you are on inactive duty training (IDT). The Marine Corps must show you were subject to the UCMJ both when the offense happened and when discipline is imposed. NJP can be given during IDT. Courts-martial generally require you to be on active duty first. If an offense from your AD, ADT, or IDT is not caught until that period ends, you can be ordered back to involuntary active duty so the case can proceed.
What it means for you
The thing that decides whether the UCMJ reaches you is your duty status, not whether you are a Marine on paper. When you drill, train, or serve on orders, you are inside the UCMJ. When you are a civilian going about civilian life, you generally are not, with the involuntary-recall exception below. Two more practical points: before you accept or refuse NJP, you have the right to talk to a military or civilian lawyer first (unless you are embarked on a vessel). And committing an offense and then leaving the drill site does not put you out of reach. You are still subject to the UCMJ even if the offense is not discovered until after you have gone home.