Looking at the previous forums, it seems like there is usually only 1 day in the 2 week period (most often the 8th or 9th day after D-Day) that Mac sends waitlist offers. For us that would've been Wednesday the 20th (8 days after D-Day). After that, it seems the first round of offers are sent out on either the 14th or 15th day, so for us probably on the 26th or 27th. Following the big round of WL offers at the 2 week mark, there doesn't seem to be an discernible pattern so it's likely that they are sent out as spots become available.
I think it's fair to say that if you don't hear back by the 28th, your chances drop substantially, although its not impossible to still get in as offers are usually made till mid June.