According to Apple's email, upgraded accounts, and their associated free storage allotments, will expire on Sept. 30, 2013, nearly two years after iCloud was released to the public in 2011.
When Apple introduced iCloud, the service gave users 5GB of free storage for photos, phone data, and other items. As a nod to former MobileMe members who were subscribed to tiered cloud data plans, Apple offered free cloud storage at their then-current tier levels for one year. The gratis storage was originally set to expire in September 2012, but Apple extended the period by one year.
In addition to the expiration date, Apple provided users with the amount of space they currently use on iCloud, allowing them time to move data off the service before the data cap is installed. It is unclear what will happen to those users who ignore the letter, though it is unlikely that Apple would erase stored data without first sending a letter of warning.