Red, Blue: 2 years, 7 months
Yellow, Gold, Silver: 1 year, 1 month
Emerald, FireRed, LeafGreen: 8 months
Crystal, Diamond, Pearl: 7 months
Platinum, HeartGold, SoulSilver, Black, White: 6 months
Ruby, Sapphire: 5 months
For reference to how long previous mainline games took to get to North America after Japan (all came out in North America first after Japan, except Crystal which came out in Australia 1 month before North America and Black/White which came out in Europe 2 days earlier).
I am curious if this is a actual sequel, if it would be considered General VI, or if it's just another expansion but with 2 games.