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180 days.... Seems like you'd just keep the original domain and 301 in place forever.
...will incoming links that are redirected to the new 301 url still pass the same (or about the same) link juice?
9. Matt Cutts: re: link juice loss in the case of a domain change: "...I am not 100 percent sure whether the crawling and indexing team has implemented that sort of natural PageRank decay."
Eric Enge comment: In a follow on email, Matt confirmed that this is in fact the case. There is some loss of PR through a 301.
It's tempting to combine the domain change with a spiffy new version of the site, but in my experience you'll retain maximum link juice and ranking power if Google's automated processes can readily see that nothing changed but the domain.