Historically it's typically been around 1-2pm. However this rMini situation has been kinda crazy, it's like Apple is shipping them out sporadically and stores are throwing them up online whenever they arrive. My local store showed nothing all day yesterday until about 5:30pm they showed stock of all wifi models. Bummer is I'm waiting for a Verizon LTE unit and my store has yet to receive a single LTE model whereas many stores across the US received LTE units yesterday...
All of that to say, at this point you should probably be checking every 30 min at least if you want a chance to get one in store.