More thoughts on how long until Part 2!
It occurred to me today that maybe the best way to estimate how long would be to see how long it takes for Legendary Pictures specifically, since they're the ones making it, to produce a sequel. Since 2005, Legendary has produced six sequels and one in-house reboot:
The Dark Knight,
The Hangover Part II,
Wrath of the Titans,
The Dark Knight Rises,
The Hangover Part III,
Man of Steel, and
300: Rise of an Empire.
Batman Begins (2005) > The Dark Knight (2008) > The Dark Knight Rises (2012)
That would be 2.5 years for BB and about 3 years for TDK. The time frame between TDK and TDKR (four years) is deceptive, because between those two movies Christopher Nolan made Inception (2010). Writing and pre-production did not begin in earnest on TDKR until 2010, so actually only 2 years.
The Hangover (2009) > The Hangover Part II (2011) > The Hangover Part III (2013)
These are comedies and smaller scale, so they may not be totally applicable. Even still, 2 years each.
Clash of the Titans (2010) > Wrath of the Titans (2012)
These are probably the most comparable, considering subject matter and extent of FX work. About two years.
Superman Returns (2006) > Man of Steel (2013)
Comparable levels of FX work, I think, and tied for the longest gap between any two (7 years). Like the gap between TDK and TDKR, though, the gap is deceptive: development didn't begin until 2008 and a director wasn't even hired until 2010.
300 (2007) > 300: Rise of an Empire (2014)
This one also took 7 years between films. They announced they were working a sequel in 2008, but it looks like production was delayed by how long it took Miller to write the story and when Snyder left to make Man of Steel in 2010.
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Thus, excluding Superman and 300, Legendary seems to average about 2 years between the release of a film and the release of its sequel. At that rate, we could expect
Godzilla 2 some time in Summer 2016.
If the sequel followed the example of Superman and 300, though, we would not see a sequel until as late as Summer 2021.