In Buenos Aires you're dealing with a few different things -- a much more rapid pace of speech (especially compared to Mexican spanish, but also compared to other areas of Argentina), the Porteno accent, the use of vos (which isn't employed throughout Argentina either), and an awful lot of slang and vocabulary that you won't encounter in Mexico, and some that you certainly won't encounter on global news. Watching local programmes will help, but you really need local ones, anything that is on the cable channels and dubbed is usually dubbed into Neutral. Local news programmes help as you'll pick up more local vocabulary but the pacing is usually more clear. If you really want a challenge try to keep up with Intratables where you can have up to 15 people shouting over top of each other.
A lot of it is the pace, people speak very rapidly. Keep the radio turned on if you're in the car / at home as well -- Ari Paluch is interesting, Perros de la Calle if you want lots of slang. To come to terms with the local speak Radio and TV can be your friends. I can't tolerate most of the soaps though, and you need to limit yourself to the local ones to get the Porteno and other Argentine accents down.
I've been here 9+ years and still working on my Spanish, some days it can be down right atrocious, sometimes I feel like I'm totally on top of it. Fluency means different things to different people, and it can truly depend on context. If you're want to be able to deliver a technical lecture in Spanish at a university level you can be talking years, if you mean social fluency, well frankly it depends how often you're in a spanish environment and how many hours of the day you can dedicate to it. If you ask me, it's not a months long practice, it's years. People often say 6 months until fluency, but really I have yet to meet someone who has achieved this. I know there are polyglots who claim otherwise, but for most adults it is a much longer journey. The trick to me is to get over your own ego, allow yourself to fail and to sound like an idiot because the more time you spend speaking another language, the more exposure you have, the more adept you will become.
