I had a similar situation when I selected my RUC's earlier this year. Selected Saturday around 7AM and was able to use an RUC for my LAX-ATL mid day flight. So IME it's about 2-3 hours until the certs are available.

I'd select them ASAP and then call Delta back about 90 minutes before your flight. And you never know, you might get the medallion upgrade and save one of your certs.