It is not clear to me what you mean about "the premise" but I can answer your question about the statute of limitations.
Because you have posted this is the "Federal Crime" section, I am assuming that the person has been charged under Title 21, United States Code section 846. The statute of limitations for a federal drug conspiracy under that provision is 5 years. You should realize, however, that there are several wrinkles that can make the case prosecutable for a longer time that you might expect. The statute of limitations may not begin to run until the commission of the last act deemed to have been a part of the conspiracy. Thus, for example, money laundering of the proceeds of the sale of illegal drugs -- which would happen after the same itself -- would still be an act that is deemed in furtherance of the conspiracy, and the statute of limitations would not run until after this act. (The defendant may also be prosecuted, in this example, for money laundering even if he or she could not have been prosecuted for the underlying substantive crime because of the passage of time.) The short answer? As with most things, it depends on the specific details of the case.
Please note that my answer here does not create an attorney-client relationship between us, and you should not rely on this advice. Rather, you should consult with an attorney retained by (or appointed for) you who can consider all the details of your case and give you advice tailored to serve your individual interests.
To charge and convict someone on a drug conspiracy charge, all the government needs to prove is that the defendant did one voluntary act in furtherance of the conspiracy. The defendant does not need to know the full extent of the conspiracy only that his one act will be in furtherance of something illegal. The act he performs in furtherance of the conspiracy may even be legal (like renting the getaway car), but he could still be held criminally responsible.