How long is the refundable period?

It is valid from immediately after payment to 23:59:59 365 days after the payment date.

The payment date is the time when the transaction status (status) becomes COMPLETED.

Hence the time is different between the immediate payment and the PreAuth & Capture as follows.

Immediate payment: The time when a user makes a payment

PreAuth & Capture: The time when "Capture a payment authorization" is normally executed

Note that when Capture is executed with an amount exceeding that of PreAuth, the timing is when a user accepted the amount to be captured.