As of the moment, there is no news on when the second movie of Percy Jackson will come out. There are rumors that say that they are planning to release it either late this year or sometime in 2012. For more information, follow this link: http://www.movie-moron.com/?p=11134.
Percy Jackson 2 entered the development stage in March 2011. This will be the squeal to Percy Jackson & the Olympians: The Lightning Thief, but there is no date set yet and it will likely be at least summer 2012 before it is out, maybe much later.