Welcome aboard! To your questions, and as parvati mentioned, a lot will depend on your job and your motivation to immerse. Living on a FOB in the current risk-averse US Army environment will limit your ability to get out and really work with the host nation in the target language. If you're lucky enough to be outside of a HQ bubble and really able to get out and about, then school, at least the US Defense Language Institute, will have prepared you well.
I've taken DLI courses (Dari) both times before I deployed to Afghanistan (Kabul). 4 month short courses for Afghan Hands immediately prior to deployment. It's enough to be able to converse in basic discussions in basic tenses (simple present and subjunctive, intransitives, active/passive sentence construction, negative conjugation, past progressive, present perfect, future progressive, etc...) but not in any work-related relevant vocabulary. 4-8 months isn't enough in my limited experience to build enough vocab to deal with heavy COIN issues. Basic security things, police/military ops, etc...and of course normal vocab at the 1+ level, but not governance, COIN, political, economic vocab.
But back to your point, training prepared me well to break the ice with Afghans, get them comfortable with my friendliness and openness, and show them that I'm making an effort...which is very important towards relationship and trust building. Not much more capability than that though. And remember the Dari (at least in DLI) will be the formal, academic language versus the spoken, colloquial language. That'll be amusing for locals to hear you speak but it won't be any kind of roadblock.
Ultimately, how useful it is will depend on you. How much you study outside of school and how much you push yourself to immerse once you get there (if you're able to). But the formal military schooling and syllabus at DLI is constructed well to prepare you to start fairly capably in the target language and build from there.
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