Very rarely have I seen it available. I fly that route 6-8 times a year, and Business is always packed. Actually I can only think of 1 instance where I have seen it, after searching numerous times to try and bring the significant other along for a trip.
Certain times of the year, if you don't book 3 months out, you are SOL for a United C class fare. I have had to fly Copa (Recliners, Stop in Panama, and a 4 hr Layover on the way back) or American via Miami/DFW. So little chance they are releasing them to awards.
Why United only has one flight servicing EZE, and American can support 1 to DFW, 1 to JFK and 2 to MIA is beyond me. Only conclusion I have come to is there isn't enough Economy demand to fill a second plane to IAH, and the future demand appears to be questionable