A trademark is valid for a statutory period of ten years. This count runs from the date of filing with the United States Patent and Trademark Office, though this comes with the caveat that it takes approximately one year before the USPTO issues a final registration certificate.
This referral back to the filing date means that protection could exist and at the same time not exist as a determination happens after the effective date of the trademark. For example, a company files a trademark, from the date of filing there is protection. However, the USPTO determines that the trademark violates the Trademark Act and denies the application fourteen months after filing. Thus, there was never protection granted, though the company could have been using the trademark for the time the USPTO was checking the application.
Companies are not like individuals though, companies can last eternally if treated appropriately. This means a ten year period may be insufficient for the company holding the trademark. There is the opportunity to continue to renew the trademark near the end of the period. Specifically, the renewal application must occur within one year of expiration. If the trademark has continued to be used and such use can be established, a renewal may be granted.
This essentially means a trademark, if renewed and regularly used in the commerce, could last forever. This is, of course, a determination to be made the company, in the sense that a company could determine that, for a variety of reasons, a trademark could be less useful. For example, think of companies that have rebranded, where they have changed their name, colors, or iconography. Because a trademark can expire, it allows companies to change how they are perceived in commerce.