Webwork - 4:47 pm on Jan 6, 2012 (gmt 0)
There is no relationship between SEO and the expiry date of a domain name.
Unless you code (endlessly) any SE's ranking alorithm - OR you have datasets the size of the NSA - it's rare than anyone can (or should) say with certainty that any given variable has no role in ranking. There's ranking algos, spam filters and heaven knows what else that come into play.
I believe, not so long ago, that extended periods of registration were considered one "slight" indication/variable/signal of a website's non-spammy intention, i.e., a small signal that a website's operator "was in it for the long haul". Was there ever an "official" statement that such was the case? Not that I recall. Were there "hints"? I kinda recall overhearing comments suggesting length of registration might play some small role - maybe in initial sandboxing, maybe in triggering spam filters, . . . Maybe, maybe, maybe. Usually from people with the knowledge to say "yes" or "no".
I wouldn't extend a registration for a decade if my sole motivation was SEO . . . unless, perhaps, I was in an uber competitive space. I would extend it, however, as a defense against an inadvertant drop, to beat a price hike, etc.