Originally Posted by xliioper

eCredits can currently be used for flights through 2024 as part of Covid credit extensions -- https://www.delta.com/edocs/redeem-documents

In normal times, eCredits from non-refundable domestic fares are subject to the following (found in the fare rules) -- "TICKET IS VALID FOR 1 YEAR FROM THE DATE OF ISSUE AND ALL TRAVEL MUST BE COMPLETED WITHIN VALIDITY PERIOD. IF EXCHANGED WHETHER TRAVEL WAS STARTED OR NOT THE TICKET MUST BE REISSUED AND ALL TRAVEL COMPLETED WITHIN 1 YEAR FROM THE ORIGINAL DATE OF ISSUE. "

Non-refundable international fares are a bit more nuanced.