1996 was Red and Green released.
1999 was Gold and Silver released.
2002 was Ruby and Sapphire released.
2004 was FireRead and LeafGreen
2006 was Diamond and Pearl released.
As I has shown here, it takes 2-3 years between each duo main games. This year has it gone 2 years since Diamond and Pearl was first released, which very likely could mean that the next duo (Gold and Silver remakes, anyone?) could be announced this year, and release this or next year