Criminal prosecution is regulated by rules of procedures and statutes. In any prosecution, there is a length of time between the commission of a crime and latest possible date that a prosecutor may file charges. This is called the statute of limitations or SOL for short. For example, in conspiracy or racketeering cases, the statute of limitations is 10 years in federal law, however; in murder cases there is no statute of limitations.
Since your question is listed under the header of federal case, I will provide analysis under the United States Cade. Each state has its own statute of limitations which comes into play when the charges are being prosecuted by state or municipal prosecutors.
Under Section 3282 of Title 18, United States Code, "(e)xcept as otherwise expressly provided by law," a prosecution for a non-capital offense shall be instituted within five years after the offense was committed. This is the section which applies to most drug charges, except drug-related homicides or conspiracies.
Once the statute of limitations runs out, the person cannot ever be charged with that offense. Thus, a person arrested by the DEA or FBI on a drug charge but who is not immediately charged (or indicted by a grand jury) would have to wait five years until he or she is in the clear.
However, as with anything in the law, there are some exceptions which can “toll” or extend the limitation period. One example among many is if after arrest, the person flees the jurisdiction to avoid prosecution or jumps bails or escapes from custody before being arraigned.
The previous answer gave you a lot of good information, particularly on federal criminal law.
As you probably realize, your question cannot be answered with specificity because you did not specify to what drug offense you are referring. Generally speaking, you must look to the Revised Code of Washington, Section 9A.04.080: Limitation of actions. You can find it at this weblink: http://apps.leg.wa.gov/RCW/default.aspx?cite=9A.04.080 .
In general, most felonies in Washington State law have a three year limitation on prosecution from the date of the alleged commission of the offense; misdemeanors have a one or two year limitation. Study the provision of the Code on the weblink above for an exact answer to your question.
Semper Fi, Patrick McLain (www.patrickjmclain.com)
It would be hard to improve on Mr. Lustig's answer. I reply just to emphasize that there are many many exceptions to the SOL defense in criminal law; do not assume that because a certain time limit has passed you are home free. Even if there are drug charges under federal law with a 5 year SOL, a prosecutor can extend that by adding a gang or conspiracy allegation. You can never be totally sure.