Hi. Can I trade in an iphone at anytime for payment?

You won't get a cash payment if that's what you mean.

You will get a gift card you can use towards the purchase of something else from Apple.

You can start a trade-in process at any time yes. If you use the Trade In during the purchase of a device, you will have 14 days after you receive the new item to turn in the traded in device to complete the trade in. If you take longer than that, the trade-in is cancelled.

Yes and if it qualifies you get an Apple gift voucher, not cash.