If you are talking about the welcome bonus MQMs, mine posted a day or so after my first purchase went from "pending" to "actual," which was a couple days after I made the purchase. So 3-4 days. No bill has yet posted, but the 10K MQMs are in my Delta account.
[added] Since you asked for definitive I looked it up. Purchase Dec 7, stayed "pending" I think until Dec 10, and on Delta the transaction was Dec 11 "activity date" with a Dec 12 "posting date." Four days, plus one to show on my Delta account.