Armed robbery is a serious felony and many states structure their statutes of limitations accordingly. They are at a minimum several years, and often, without limitation entirely.
Statute of Limitations
The statute of limitations is the time period within which civil action or criminal charge must be filed with a court after a triggering event such as an injury or crime. If the statute of limitations lapses, the legal action is forever barred. In criminal cases, there are a few exceptions to this rule, which vary by state. In general, the criminal must be "catchable," meaning in-state and available to be prosecuted. Thus, a catchable criminal must be charged within a given time or they can not be prosecuted.
Armed Robbery
All robberies are considered violent felonies. The presence of a weapon, however, increases the violence level of the crime committed--even if the weapon is not used. Most states upgrade the class of felony, which represents a level of seriousness, whenever a weapon is present.
In Application
The statute of limitations for armed robbery varies widely. Each state writes their own laws and penalties; thus, each state decides the statute of limitations. Pennsylvania, for instance, places a five year statute of limitations on armed robbery; Georgia, on the other hand, generally allows four years---unless DNA evidence establishes guilt, in which case there is no statute of limitations---you can be convicted as long as you are alive. Ohio provides 20 years. Many states, incidentally, have no statute of limitations on armed robbery, in view of the crime's violence and seriousness.