Reposting since it was right at the bottom of the last page
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Let's try to give a better perspective of how things could go down in terms of announcement-to-unveiling time period, by looking at past events' announcements.
Nintendo 3DS Preview Event
http://kotaku.com/5715436/nintendo-to-reveal-3ds-details-on-january-19
Invites going on December 20th, announcement on December 20th, event on January 19th
Time between announcement and event - 31 days (Christmas Holidays in the middle could've influenced the just mentioned time period)
Nintendo 3DS Conference 2011
http://www.bloomberg.com/news/artic...-osaka-amid-speculation-over-event-next-month
http://www.neogaf.com/forum/showthread.php?t=442041
Invites going on August 22nd, announcement on August 23rd, event on September 13th.
Time between announcement and event - 21 / 22 days
Wii U US Preview Event
http://kotaku.com/5936634/looks-like-well-know-the-wii-us-launch-date-and-price-by-september-13
Invites going on August 21st, announcement on August 21st, event on September 13th.
Time between announcement and event - 23 days
PlayStation Meeting 2013
http://www.neogaf.com/forum/showthread.php?t=511056
Invites going on January 31st, announcement on January 31st, event on February 20th.
Time between announcement and event - 20 days
Xbox - A New Generation Revealed
http://www.neogaf.com/forum/showthread.php?t=546448
Invites going on April 24th, announcement on April 24th, event on May 21st.
Time between announcement and event - 27 days
PlayStation meeting 2016
http://www.neogaf.com/forum/showthread.php?t=1260606
Invites going on August 8th, announcement on August 8th, event on September 7th-
Time between announcement and event - 30 days
EDIT: Unfortunately, I could not find when the Nintendo Fall Conference 2010 (which was slated on September 29th, 2010) was announced. If anyone remembers it, I'll gladily add it in my post.