How long do trademark rights last?

First, the continued validity of a trademark registration is dependent upon actual use of the trademark. In this sense, trademarks are not true property rights in that an express, or circumstantially proven, permanent intention to abandon the trademark is sufficient grounds to cancel a federal trademark registration.
Trademark rights can also be lost through dilution. Dilution commonly occurs where competitors adopt very similar trademarks and the trademark holder takes no action to defend the mark.
Another means of losing trademark rights occurs when the trademark becomes so identified with the particular, precise goods to the point where the mark loses its distinctiveness and instead becomes a generic descriptive term. An example most people are familiar with is aspirin, which was originally a trademark (owned by Bayer), but became so identified with acetylsalicylic acid tablets that it eventually turned into a generic term. Other companies are fighting to keep their trademarks from being found generic. These include Xerox ® for photocopiers.
An actively used trademark can continue to exist, increase in value to the company, and increase the sales and profits of the company for an unlimited period of time.