Trademark rights can potentially be unending. However, how long a trademark lasts depends on a few factors.
In the United States, trademarks can last indefinitely as long as they are used properly and renewed regularly. After 5 years the trademark owner must file a declaration of use and must pay the appropriate fees. Then, after the 9th year the trademark owner must file a declaration of use and a declaration of renewal and pay the appropriate fees. To maintain your trademark rights a declaration of use and a declaration of renewal will need to be filed and fees will need to be paid every subsequent 10 years.
It's important to note that trademark protection requires active use of the mark in commerce. If a trademark owner stops using the mark for an extended period of time, they may lose their trademark rights. This is known as "abandonment."
In addition to proper use and renewal, trademark owners must also actively monitor and enforce their trademark rights to prevent infringement by others and to keep the trademark from becoming generic. Failing to enforce trademark rights can weaken the strength and value of the mark over time.
Overall, with proper use, renewal, and enforcement, a trademark can provide long-lasting protection for a brand or business.