FEX stands for "Field Exercise". It's the fourth one of the POI (Program of Instruction) at the BOC (Basic Officer Course).
Then I looked at your profile and realized you are a Marine captain, how do you not know what a FEX is?
more like 3-4 days, maybe they just took longer to pick up things in "Old Corps" Semper Smilies :icon_smil
I'm in Echo, and as of right now FEX IV is cancelled. Apparently it is being replaced by some training event in Norfolk, but they haven't told us what it is yet, or even if it's a sure thing. The rumor is that it's an amphibious landing, but nobody knows for sure yet.