Technically, if you follow all of the requirements and continue to use a mark it can last forever. For a trademark registration to remain valid, an Affidavit of Use (Section 8 Affidavit) must be filed: (1) between the fifth and sixth year following registration, and (2) within the year before the end of every ten-year period after the date of registration.
The registrant may file the affidavit within a grace period of six months after the end of the sixth or tenth year, with payment of an additional fee. The registrant must also file a Section 9 renewal application within the year before the expiration date of a registration, or within a grace period of six months after the expiration date, with payment of an additional fee.