The times mentioned in earlier posts are, I think, for App And Media stores refunds and may not apply in your case since you are asking about an iPhone. What you probably need is:
Go to:
Returns & Refunds - Shopping Help - Apple
then scroll down to “Refunds”
If you paid by credit or debit card, refunds will be sent to the card-issuing bank within five business days of receipt of the returned item or cancellation request. Please contact the card-issuing bank with questions about when the credit will be posted to your account.
Refunds on products purchased from Apple using an Apple Gift Card (or previous generations of Apple gift cards) will be issued to you as an Apple Gift Card by email. If you’d like us to send you a physical Apple Gift Card by mail, ask the representative helping you.
Products paid for using your Apple Account Balance will be refunded to your Apple Account Balance.
I provide you with the USA version of the article and you may wish to follow the link and see what it says for your country.