Federal investigations feel like they take forever. Agents come, they talk to you, perhaps they execute a search warrant or send a grand jury subpoena to your business. There's a great flurry of activity. You hire a lawyer, then circle your wagons.
You'd like to know how long this can take.
The hard part is that, generally speaking, you're waiting for something bad to happen. Many United States Attorney's Offices do not let you know that they've decided not to go forward with criminal charges. The policies vary by office, but, generally speaking, the way you find out that you're no longer under investigation is that the statute of limitations runs.
Or, worse, you find out that you're no longer under investigation because the government brings criminal charges against you. The Statute of Limitations For Federal Crimes
For the vast majority of federal crimes, the charge has to be brought within five years of when the crime was committed. The grand jury indictment is the official charging document, so what that means is that the indictment has to be returned by the grand jury within the five-year period.
There are a federal few crimes that have longer time limits: art theft is twenty years, arson is ten years, defrauding a bank is ten years, and violations of immigration law get ten years. Kidnapping or sexual abuse of a child has to be charged either during the child’s lifetime, or within ten years (whichever is longer).
There is no time limit on capital offenses (murders that are serious enough that you can get the death penalty) or certain dangerous acts of terrorism. The time limits also don’t apply if the government can prove that the person charged was actively avoiding prosecution – for example, by leaving the jurisdiction and hiding when he or she knows police are looking for them.
There are also some odd rules around statutes of limitation and cases that require evidence from foreign governments. If the federal prosecutor has to get information under an MLAT - or Mutual Legal Assistance Treaty - then they get some extra time to get an indictment.
And, unfortunately, many federal crimes are also state crimes. So, if you're out of the woods with the feds, what they're investigating may also be able to be prosecuted by a state prosecutor. Normally that doesn't happen - state prosecutors tend to not be interested in the same kinds of cases that federal prosecutors are - but sometimes a federal prosecutor who feels angry can get state attention to a case.
All of this is to say that if you're worried about a federal investigation, and you're hoping you get an all clear sign, you may, unfortunately, have to wait a while.
Probably the best sign that you aren't going to be indicted - that the investigation is over - is when the AUSA who was working the case moves on. Either to another job, another office, or some other kind of case. It isn't iron clad, but it gives you a sense that you probably are moving farther out of danger.