It depends on the store. Yesterday, they had me standing in the same line as everyone wanting to buy the iPhone. So it could be very quick, or it could be hours. It is best to go in the evening.
Not longer than 45 minutes for me. I showed up at opening time(8am) and they had two lines going. The pre-order line was only about 20-30 people while the walk-in was at least 250-300. The apple store I went to separated the store in half for walk-in and pre-order and had at least 10-15 employees on each side processing phones. They would take 8-10 people at a time from each line as people were finishing up their purchases.
I picked up at the downtown San Francsico store yesterday and waited in line for 1 hour. So much for my lunch...no coffee or treats being passed out that I heard were available at certain locations. Only Apple employees and police being treated to bottled water.
20 min max, in fact this morning they let me in before the store opened and took care of me - ft lauderdale galleria store -very impressive - not a reserve but a full pickup paid for online to store...