Thanks to MLBTrade Rumors, we find that the projected cut-off date for Super Two status (in which a player who is promoted for the first time before a certain date gets an extra year of arbitration) is June 3.
So this means Paul Clemens, Brett Oberholtzer, Jose Cisnero, and Robbie Grossman will all qualify for Super-Two status, should that day come. It also means you can probably pencil that date in as the absolute earliest you'll see Jarred Cosart or Jon Singleton (who is eligible to come off the suspended list in late May).