This will vary by state. An excerpt from guidelines published by the Pennsylvania Bar Association, which quotes another ABA document, says in pertinent part:
Going beyond the issue of what is the client's property, and directly addressing the
retention and destruction of files, the American Bar Association Committee on Ethics and
Professional Responsibility has stated:
How to deal with the burden is primarily a question of business management, and
not primarily a question of ethics or professional responsibility. A lawyer does not
have a general duty to preserve all of his files permanently. Mounting and
substantial storage costs can affect the cost of legal services, and the public
interest is not served by unnecessary and avoidable additions to the costs of legal
services. But clients and former clients reasonably expect from their lawyers that
valuable and useful information in the lawyers files and not otherwise readily
available to the clients will not be prematurely and carelessly destroyed, to the
client's detriment.... We cannot say there is a specific time period during which a
lawyer must preserve all files.... Good common sense should provide answers to
most questions that arise.
ABA Standing Committee on Ethics and Professional Responsibility, Informal
Opinion 1384 (1977).
A thorough reading of the articles published in legal professional journals and periodicals
on file retention and destruction reveals the following recurring, common
Lawyers and law firms should develop a detailed file storage, management and
retention policy and should follow the policy consistently. The policy should address the
return to the client upon the conclusion of the matter, case or transaction of all original
documents such as business records, deeds, estate papers, title insurance policies and
abstracts, and papers personal to the client. Such a policy should further include the
direction of a closing letter to clients which notifies them of their entitlement to take any
documents not previously furnished to them and may advise of a date on which the file
will be destroyed in a manner which will protect the client's confidentiality.
The decision on how and when to destroy part or all of a file should be made by a
lawyer, not by an office manager, paralegal or other staff person.
This 2003 document from the ABA discusses the topic at length, as well.