In Illinois, there is no limitation period for "forgery" under the Illinois Criminal Code of 1961. See Section 3-5(a)(1). That means, if the prosecution decides the facts warrant a charge of forgery (versus another felony), then he or she can bring the charge at any time in the future.
If another crime is charged, and the statute defining the crime does not provide otherwise, then the general statute of limitations for felonies is three years from the commission of the offense. Section 3-5(b).
NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.