I'm pretty sure it was officially announced in 05, and the first trailer (confirming Wario, Pit, MK, ZSS, and Snake) was shown in 06.
Super Smash Bros. 3DS/WiiU was announced at E3 last year, about 3 and a half years after Brawl's release, and development began this year.
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darkx congratulates SuperNiceDog. Now go find Nice Dad and email Hungry Shark.