As far as we are aware, the BAcC retains all the materials relating to complaints about a practitioner's conduct or behaviour indefinitely. There are very few each year, so there is no logistical problem about keeping them. The logic behind retaining them indefinitely is that if a pattern emerges over time then even where a complaint is not pursued or no ruling is made.
The only minor complication would arise if the complaint were not to be about the conduct or performance of a practitioner but about the consequences of a treatment, i.e. the basis for an insurance claim. Along with most other healthcare professions members of the BAcC are required to retain their treatment notes for a minimum of seven years (or seven years after the age of majority for a child under sixteen, so theoretically as much as twenty three years) for insurance purposes. The insurers rarely have claims arising after three years from point of treatment, and although the insurance cover lasts indefinitely if the member was insured at the time, there are a number of data protection issues about holding on to historical information about past patients.
This might mean that although the BAcC may have a record of a complaint eight years ago and all of the attendant statements from the time, the primary record may no longer exist. In most cases this is not so; unless we are pressed for storage space we tend to hold on to files for much longer because people do return after a decade for treatment. There does come a point, though, where it is unreasonable to be holding personal information taken a very long time ago, and where there can be no real justification for hanging on to material.