The short answer is forever.
Trademarks as well as service, collective, collective membership, and certification marks all last as long as the owner keeps using the mark, and keeps paying renewal fees.
When a mark is first registered, it will live for ten years from the registration date. A renewal fee and affidavit of use is needed to keep the mark alive. If the renewal is not submitted upon the tenth anniversary, the mark is declared dead, but may be revived within six months for a surcharge.
Marks are eligible for renewals every ten years, indefinitely.