It depends on the charge. For certain offenses there is no statute of limitations. Generally, most federal crimes have a statute of limitations of five years, although there are exceptions that apply for certain crimes, when the wanted person is a fugitive, crimes against certain victims, etc. you should speak with an attorney for more specific answers.
Please don't forget to choose the "best" or most "helpful" answer. This response does not create an attorney client relationship. The information provided here should not be relied upon and is not a substitute for legal advice received by an attorney that you have retained for your specific case. If you are actually involved in a case pending in court and you have an attorney, you should speak to your attorney and follow his/her advice. The information provided here is for general purposes only.
Speak to a criminal defense lawyer who handles federal cases in your area. Your questiojn does not have enough information to give you a good answer.