@pete-s In the US they tend to say "as short as possible." Email is always a legal quagmire and the best thing to do is to delete is as quickly as possible. Which, of course, can't be that fast. So we are generally talking 1-2 years. But you rarely want to keep it longer not because it likely contains details of people breaking the law, but because a legal discovery request is extremely expensive and a great way to attack even otherwise honorable businesses. It's a huge cost you can leverage against someone that they can only reasonably mitigate by not having much email to go through.
Man - that would be so awesome. But even if management did agree that - you'd have people that would be looking for ways to maintain the data for a much longer period - like printing and saving in a cabinet.. shudder.
I like many of the replies I get about cleaning out email. "Why, its free!" "Why, my 50 GB of email is nothing when we have 16TB drives for $200" "Why do I have to remove email older than 13 years, it isn't hurting anyone" "Why would I do that, I may need it later (Medicare Newsletters prior to 2010)" and the list goes on and on.
Then my next question is - if something is so important that you need to keep it - why is it in email in the first place? Why can't you get that data someplace else more related to whatever it is you're saving it for? (That said, I realize that other documentation for something simply don't exist).
Don't you dare get me started down this path. I had HUGE arguments about this with an ex-employee over the period of 10 years. The user could not/would not understand her email box is not a document database / DMS. The last I counted, she had over 300 different nested folders in her email.
Now that the user is gone, their mail copied to a shared mailbox for management to hunt/search and waste their time with if they choose.
It probably easier to have retention policy in place from the start.
If you know email retention is time-limited, you'd have to come up with some other way to store things.
But some people are just hopeless no matter what...