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Depends.
If it weren't for hacking, we wouldn't have known about Arceus in Gen 4 until after 3 years.
And Genesect/Meloetta were revealed about 2 years after Black/White.
Gen 4's events weren't even revealed when Diamond/Pearl came out. It wasn't until Platinum and close to HeartGold/SoulSilver, IIRC.
Victini was the only event that was immediate, with Genesect & Meloetta not available until Black 2/White 2 and Keldeo was in-between.
As far as 3rd generation, Deoxys was revealed with FireRed & Leafgreen and Jirachi was with Colosseum or around then.
So the earliest we'll have one revealed is within a few months. Maybe early 2014 or late December, because it will probably be in the next Pokemon movie and revealed in Japan with a trailer. The last one (assuming there's more) could be revealed anywhere after that up to 2015 or 2016 unless Gen 6 is short.
We probably won't get some until potential sequels or remakes, however, even though they'll be in the code of X & Y.
