Search Engine Optimization is mostly an art, based on a little science and the diverse experience of the web community. That just means that with many SEO issues, you're stuck with an educated guess rather than a definitive answer.
One such issue is the length of your domain name registration. Some Registrars say if you purchase your domain for a longer period, such as 5-10 years, it will help your search engine ranking. Is that just a sales pitch? Maybe, maybe not.
Google hasn't been helpful. John Mueller of Google said determining registration length for many domains "would be pretty hard, and likely not worth the trouble." Matt Cutts said "To the best of my knowledge, no search engine has ever confirmed that they use length-of-registration as a factor in scoring." But later said, "My short answer is not to worry very much about that...", "...make great content, don’t worry nearly as much about how many years your domain is registered."
But what about the patent? Filed in 2007, the patent states, "Certain signals may be used to distinguish between illegitimate and legitimate domains. For example, domains can be renewed up to a period of 10 years. Valuable (legitimate) domains are often paid for several years in advance, while doorway (illegitimate) domains rarely are used for more than a year. Therefore, the date when a domain expires in the future can be used as a factor in predicting the legitimacy of a domain and, thus, the documents associated therewith."
Google is not explicitly denying that the length of domain registration is a factor in rankings, and they have a patent to do so. So form your own opinion about whether the length of your registration affects your ranking. But the fact is, domain registration is an inexpensive business expense. When you consider the importance of high ranking, it seems a no-brainer to extend the length of your registration beyond a year or two. It just might help, and it won't hurt. If it does help your ranking, you benefit. If it doesn't help at all, at least you don't have to worry about renewal for awhile.