Videogirl on 04 June 2008
There were 18 months between Ico release and Shadow of the Colossus first rumors : some screens leaked begining of 2003, but the game was only annouced 18 months later September 2004 and released in Japan one year after that (October 2005).
It means 4 years between their first and second game.
Since Shadow was released end of 2005 you can expect the next one for end of 2009, but the game will most likely be officially announced either at E3 or TGS this year, thus giving Sony time to hype it.
