I called the Delta refund line (800-847-0578, available 8 am to 7 pm) today for 2 refunds. After a 2-hour wait (no call back option), my friendly agent told me my first flight (domestic) was eligible for a refund. I told her in a courteous manner that my expectation was that Delta would follow the DOT's May Enforcement Notice and provide a prompt refund, which was defined in the notice as 7 days. She said the refund was initiated, and the money would be back on my credit card within 7-14 days (2 days is my experience with previous immediately-initiated refunds).
On flight #2 (international business class), she told me that the system would not allow her to initiate the refund, and she was going to have to fill out "the long form" (which is code for "slow refund"); she said the refund would take 30-90 days. I told her, again in a respectful tone, that unfortunately that meant I would have to file a complaint with the Department of Transportation at Day 7 if there was no refund. She put me on hold and came back to tell me that she was able to go ahead and initiate the refund and that the money would be back on my card in 7-14 days.