The minimum storage is one year.
The rest depends on the firm and their policies.
I personally recommend 10 years, simply because it's not that much bother to do.
Some professional firms have longer liabilities and may need 12, 15 or indefinite archival access.
Edit: I should quantify that email retention is majoritively to do with legal liabilities. If the company's statute liability ends after 7 years then keeping emails for 8 years is perfectly adequate.