I'm waiting on my trade in return box as well. They told me I have 15 days to return my trade in once I get my note 8. However, like I said it looks like my return box has not even shipped out to me yet. I guess I will be giving them a call sometime today. They also told me that I can not turn in my trade in to a local store. But yea, I think you have 15 days to return your trade in.
Edit: To be more specific on the process...they send you a small box with a pre-paid label to send it back to them. Just make sure (if you have an iPhone) that find my iPhone is disabled and that you have completely erased all of your personal stuff.
i got my return box mailed to me last week.