A federal trademark registration with the USPTO lasts for a term of 10 years. Once the registration certificate is issued, maintaining the trademark is relatively easy. The registrant must file a document called a Declaration of Use under Section 8 between the fifth and sixth year after registration, which states that the registrant is still using the trademark in commerce.
The registrant may thereafter renew the trademark by filing a Application for Renewal under Section 9 between the ninth and tenth year after registration, and every subsequent 10-year period. If the applicant does not file the Section 8 or 9 affidavits in a timely manner the trademark may be deemed abandoned by the USPTO and may be subsequently registered by another party.
Generally, my blog articles are quite a bit more detailed than this one, but I get asked this question so often that I thought it warranted a stand-alone blog post. For more information on trademark maintenance you can visit the USPTO website or our Trademark Registration page.