If VoDT releases summer of 2010 and they are discussing releasing a movie every other year then we should see SC in 2012, HHB in 2014, MN in 2016 and LB in 2018.
If they can speed up production enough to put 1 1/2 years in between films then we can see SC at Chirstmas 2011, HHB in Summer 2013, MN at Chirstmas 2014 and LB Summer 2016. That would mean 10 years for seven movies, which is not that much different than the Harry Potter films (Sor. Stone in 2001 - DH Pt. II in late 2010-early 2011)
If the HP films can keep making revenue through the end then the Narnia films should be able to do even better. Since Disney is interested in money more than anything else as long as the films make them a good profit then they will continue to produce them.