The Elder Scrolls III: Morrowind - May 1, 2002
(6 years after TES II)
The Elder Scrolls IV: Oblivion - March 20, 2006
(4 years after TES III)
The Elder Scrolls V: Skyrim - November 11, 2011
(5 years after TES IV)
The Elder Scrolls VI (14 years and counting)
At this rate, TES VI might actually be one of the longest waits for a sequel in gaming that we'll see by the time it comes out.