So I renewed my visa on Tuesday and it was a pretty painless process. Except for the fact that I didn't realize I needed to bring a photocopy of my passport (the front page and
the stamp of entry). The whole process took about an hour + another hour or so to walk to retiro and make photocopies, I got there at ~8:15am and got back around ~10:30.
Here's how you do it. When you arrive at immigrations, walk down the driveway all the way to the back "al fondo" of the property and then turn right. Walk a littler further, you should be looking for Edificio 6. There may be a line, but once inside, there should be someone in the front and tell him you want to "extender mi visa turista" and then he'll give you a paper slip aka "turno". Go to the far right, "a la derecha al fondo" and there will be an entrance to another room. Inside that room, go all the way to the right where there will be a sign that says "Prorroga de permanencia". That's you. Wait to hear your number called. (Note that the "turno" displayed overhead in the front of the room doesn't pertain to you. Your turno will be called) Meet with the immigration officer, give her your passport and photocopies. She'll check some stuff then give you a document. Walk outside the room with the document and you'll find the cashier immediately on your left. Pay $300 pesos in cash, get a receipt, and then bring it back to the same immigration person. She'll submit everything (documents and your passport) to some other immigration lady for review while you wait. Once everything has been checked, you'll hear your last name "apellido" called. Go and pick up your passport with a fresh stamp.
Couple notes on my experience:
Nov - Entered Argentina
Dec - Made a trip to Uruguay, got a new stamp
Feb - Got an extension until May
By the way, the extension grants you 180 days from your last entry, not 90 days from the day you are granted the extension. So my advice is to take care of this sooner than later, no reason to delay.
The room on the right