Trademark rights can last forever, as long as you continue to use the mark in connection with goods or services in commerce. Courts deem a trademark to be lost through abandonment if the holder does not use the trademark for three consecutive years. Actively managing trademarks to ensure they are publicly used on an ongoing or periodic basis can prevent the trademark from being canceled due to non-use.
A trademark can be lost due to genericity, or the process of becoming generic over time, if the owner doesn't promptly object to people using the mark as an ordinary verb or noun. Terms like "thermos," "aspirin," and "cellophane" were once corporate trademarks, but lost their trademark status once the terms passed into the general public lexicon. Courts include the degree to which a trademark owner has policed her trademark as a factor considered when determining whether trademark protection should be voided due to genericity.
Failing to police your trademark can result in losing the right to bring a lawsuit for infringement against someone who is using a similar mark. Delay in filing infringement lawsuits when you are aware someone is using your trademark gives rise to a legal concept known as "laches," which effectively means that you are locked out of asserting your rights for failure to act in a timely fashion. Courts consider the level of diligence you exercised in protecting your trademark as one element considered when determining whether you are precluded from prevailing in an infringement lawsuit.
Economic Losses and Damages
Failing to adequately police your trademark can result in a loss of business income, as consumers become less able to distinguish your products from those of your competitors. Failure to actively promote and protect your trademark can also result in an inability to secure damages in trademark enforcement lawsuits based on dilution -- using a similar trademark on different products that diminishes the distinctive quality of the mark. To secure damages in a dilution suit, the trademark holder must demonstrate their mark was famous by showing how the mark was extensively used and advertised.