I'd give it at least 3 hours to be comfortable.
You have to buy a ticket on one of the two (or more) airport bus lines (may be cheapest for a solo passenger). Manuel Tienda Leon busses leave every 1/2 hr and go to its city hub first before heading for Aeroparque Jorge Newbery (AEP). I'd give this trip at least 50 minutes if not 1 hr.
So add 1/2 hr to clear immigration and customs, up to 1/2 hr wait for the bus, 1 hr to AEP + safety buffer.