I have permanently changed my address, and it does seem as if the forwarded mail falls off after about a year. Temporary forwards have an end date (up to a year after the change) and permanent address changes do not----BUT certain classes of mail will cease being forwarded after a year or less. I will provide you the links that I have. I would also urge you to see the postmaster at your local post office to discuss any problems that you are experiencing with forwarded mail.
See "how long is change of address mail forwarded":
Actual address change form:
I apologize for the confusion of the website, but business forwarding is not covered specifically. However, on Form 3575, there is a choice for "individual" or "business", so the same form covers both situations. I have also verified via phone call to the USPS that the rules for forwarding (12 months for 1st class, 60 days for periodicals, 12 months for local packages. etc.) apply to business as well as personal mail.
Again, I apologize that this is not specifically referred to for business on the website.