ipstudent is right -- if the infringement is ongoing, there is no statute of limitations.
However, if the infringement is allowed to go on for "too long," then the accused infringer may be able to successfully raise a defense of laches -- that is, if they wait too long, they may be found to "give up" their right to sue in the name of fairness.
"Too long," however, is a question of fact -- there is no simple rule that will tell you what is "too long." When a defense of laches is raised, the court will look at all of the facts, including such things as how long it has been, how serious the infringement is, how "widespread" the infringement is (i.e., is it infringement only in some small town in the Midwest, or is it national or international, like on the web), and, finally, how aggressive or non-aggressive the trademark holder has been in past infringement cases.
If you believe you are currently infringing and haven't been noticed, you could take all of your facts to an attorney who might be able to give you a better idea of the situation. If you have already been sued, you definitely need a lawyer!