How registering your domain name for longer periods might help your search rankings.

I've heard that Google interprets longer domain registrations as a
signal of quality. Is it true that Google will treat my site differently if I
register or renew my domain for 10 years versus just two years?

Answer: The short answer is: It will never hurt your rankings and it might,
theoretically from time to time, help your rankings ever-so-slightly—maybe,
but probably not. Here's the deal.

A Google patent made public in 2005 referred to several ways by which
Google may distinguish between illegitimate and legitimate domains. One
aspect is the length of time a domain is registered for. According to the
patent, legitimate domains are more likely to be paid for several years in
advance, while doorway and spam-heavy domains are rarely registered for
more than a year in advance, since the owner knows they'll likely be
penalized anyway.

Based on the patent information, several domain registrars began prompting
their customers to lock-in new registrations for long periods, claiming that
doing so would help their search engine rankings.

However, since that patent was released our observations indicate that
extended domain registrations don't have much, if any, impact on
rankings. It certainly doesn't hurt, but you should not expect it to give you
any major boost in the rankings.

Even so, it is interesting to note that Google's patent application does state
that domain registration could be a potential ranking factor. But just
because something is in a patent doesn't mean that it's actually being done.
All it means is that the patent holder wants to reserve the right to possibly do
it someday.

Also relevant is the fact that Google, as a domain registrar, only has access
to the database of public (but not private) whois records. So, in theory, if you
keep your domain registration private (as many do for privacy and personal
security reasons)

Google has no easy way of knowing how long your domain is registered for.
Finally, it's worth noting that domains in the UK are prevented from being
registered for periods longer then two years at a time, making the point moot
for British sites.

So the bottom line is that it may be good business practice to register your
domains for longer, but you really shouldn't count on getting a ranking boost.