The duration needs to be extended if the secondary skin or ear infections have not been cleared yet or if the cat shows improvement but not complete resolution of clinical signs after 6 weeks. (2ndchance.info)
Most pets need to be fed S / D for 4 - 6 months for complete resolution. (lbah.com)
Moreover, the company is offering the support services for the users that are facing such issues and they are providing complete resolution of the problem. (thedevicesupport.com)