When I worked at a law firm in Colorado (and I'm sure it's the same case around the country), at the end of a trial/case, the Court sends a notice to the attorneys who filed exhibits with the Court that if they do not pick up their original exhibits (by a certain time) used for that case, the Court will destroy them. They simply don't have room to store everything on closed cases except for the legal documents files, i.e., leadings/affidavits/summons.
I can't think of any case where an attorney would not have his own copy of exhibits filed with the court. And, usually, the attorneys keep everything necessary with regard to that case, perhaps up to 7 years, if not longer. That depends on either on the firm's own desires and/or the state's requirements for how long legal documents must be kept by the attorney.
sorry, typo; that should be: except for the legal documents FILED, i.e., PLEADINGS/affidavits/summons.