Well, let's do some calculations here:
FE7 was released in America on November 3, 2003 and Sacred Stones came out on May 23, 2005, an amount of 931 days have passed in that time.
PoR was released on October 17, 2005, which makes 147 days since Sacred Stones.
RD was released on November 11, 2007 which makes 755 days since PoR.
SD was released on Februrary 17, 2009 which makes 473 days since RD.
When you average all those up, you get approximately 577 days, which lands on the day September 16, 2010.
This is just a rough estimate, but if we got a new FE, it could take that long.