If you go by US Release Dates Its:
Kanto - 1998
Johto - 2000
Hoenn - 2003
Kanto Remake - 2004
Sinnoh - 2007
Johto Remake - 2010
Unova - 2011
Unova Sequel - 2012
So I Would expect them to Anounce Gen VI Soon more or less But the Anime won't end until the Games are actually released in Japan (Thats why Sinnoh was so damn long)
EDIT: In recent years we have seen a New Main Series Pokemon Game Every Year since the Johto Remake's
You never go on US you go on japan because they come out there first Plus we are talking about the anime series.
If you look at the anime:
Indigo League: April 1, 1997 – January 28, 1999.
Orange Islands - January 28, 1999 – October 7, 1999.
Johto Journeys - October 14, 1999 – July 27, 2000.
Johto League - August 3, 2000 – August 2, 2001.
Master Quest - August 9, 2001 – November 14, 2002.
Advanced - November 21, 2002 – August 28, 2003.
Advanced Challenge - September 4, 2003 – September 2, 2004.
Advanced Battle - September 9, 2004 – September 29, 2005.
Battle Frontier - October 6, 2005 – September 14, 2006.
Diamond & Pearl - September 28, 2006 – October 25, 2007.
Battle Dimension - November 8, 2007 – December 4, 2008.
Galactic Battles - December 4, 2008 – December 24, 2009.
Sinnoh League - January 7, 2010 – September 9, 2010.
Black & White - September 23, 2010 – September 15, 2011.
Rival Destinies - September 22, 2011 – June 14, 2012.
Best Wishes - June 21, 2012 – present.
Lets recap -
Kanto to Johto took 2 Years.
Johto to Hoenn took 3 Years.
Hoenn to Sinnoh took 4 Years.
Sinnoh to Unova took 4 Years.
So we can either say that 6th gen will be out in 2014 or 2015.