In lots of industries it's not subjective at all. Certainly, CRA has rules with regard to how long you're keeping.... Financial institutions, the doctors, the lawyers, all have governing bodies that assist them in developing document-retention policies. There's a reasonableness test there, that when you reasonably no longer need that data, it's time to make it go away, unless the law says you have to keep it for a longer period of time.
Coming up with a policy for when documents should be destroyed is not a difficult process at all. Most companies have a retention policy as to how long they're going to keep documents. That's not the biggest hurdle there, for sure.