You can view any data from up to two calendar years. For example, a campaign created on January 1, 2019, will retain all of its data through December 31, 2020. After January 1, 2021, then the current data displayed for this Campaign would be from January 1, 2019, to the current date.
Note: Email insights data retention is completely separate from Acoustic Campaign data retention and the length can't be altered. However, any data synced to your Acoustic Campaign database follows the retention settings found in Org Settings > Archive Settings.