I spoke with an Allegiant representative two days ago using their online chat feature. The representative said Allegiant typically schedules its flights roughly four to eight months in advance. I was specifically asking for early May 2011 dates, and she indicated those dates should be viewable by the end of November.
I would imagine they would not give you a hard-and-fast answer, but you could always access the chat feature and ask about late March. In either case I would expect them to be viewable before too long.