It is essential to file a case as soon as possible after you have learned about fraud. The False Claims Act says that whistleblower must file a lawsuit within:
Six years from the date of the violation
Three years from the date the government knew or should have known about the fraud but in no event more than 10 years from the time of the violation, whichever is the latest.
For example, if a fraudulent action occurred four years ago, a suit can still be filed. Also, if the activity occurred seven years ago, but the government does not and could not have known about it until now, then the statute of limitations has not expired, and you can file a lawsuit.
However, if the action occurred 11 years ago, the statute of limitations has expired, and it is too late to file a suit. Also, if the activity occurred eight years ago, but the government knew or should have known about it five years ago, then the statute of limitations has run, and a suit is prohibited.