They confessed to it right away, and it was Russian propaganda outlet Life News first on the scene almost right away (they like to party). The only problem was they thought it was a Ukranian cargo plane, so they deleted their postings and all their bragging about having Buks when Life News came back with videos of dead civilians.
It was sooner than that. They immediately dispatched a squad of fighters to the crash scene, expecting to round up any crew who bailed out, as they had after they shot down that Kiev AN-26 cargo plane the Monday before (two bailed out; six died). However, when they got there, they found dead civilians bearing foreign passports. They gave the bad news to their commander about forty minutes after the shootdown, and that's when the social media boast came down.