I have used Gazelle for the past 2 years to sell iPhones, Macbook Pro's, iMacs, Cameras and the result was always the same....GREAT!
My experience is usually 2 weeks to get everything taken care of. From the time you send the box to them to the time you get the check in your mailbox is roughly about 2 weeks. Although, last summer when I sold my family's old iPhone 3GS to them (5 of them), it did take almost a month to get my check. Guess with the fact that they were so busy at that time with the iPhone 4 releasing and having so many iPhones to check out from those customers, it makes sense!
My last transaction with them was just over 10 days. They are great!