If the company has been selling a product for some time, and it was not patented. But then the original inventor decided to patent the product. You still have it in stock. How long have you got to sell this product without a patent?
There is no grace period. As soon as you receive notice of the patent (or, in some cases, the published application) - or should have received notice (i.e. once the patent is published, you are typically assumed to have received constructive notice).
For a good discussion of 'actual notice', 'constructive notice', and the issues around potential liability for infringement that occurs before a patent actually issues, you may be interested in this article by IRMI.