Data for sent campaigns is retained for a period of time specified in your license agreement. The typical data retentions are as follows:
Basic/Starter - 90 days
Professional - 450 days
Enterprise & up - 1080 days
Once a sent campaign reaches the data retention mark, all data for that mailing is removed from the database. Your analytics reports will show line-item information, but there will be no drill-down data for who received the campaign, what the design looked like, who opened, clicked, unsubscribed, etc.
Contact Support or your Client Success Manager for information about your data retention. This retention period can be extended but any data that has already been deleted cannot be recovered.
This time limitation also applies to API calls made to retrieve data.