When do sold-out K-pop concert tickets come back?

Right after the payment deadline, most of all. Yes24 cancels unpaid bookings at 23:29 and Melon Ticket at 23:59:59, and those seats return to sale.
The second-heaviest window is seven to three days before the show. Cancelled payments cluster in those two windows and keep trickling back in between.
Sold out is not over. Unpaid bookings are cancelled at a fixed time, and the person refreshing just after that time gets the seat.
Switch on phone alerts on this site and the lock screen tells you just before seats are released. It does not go by email; it is faster than email.
Cancel times differ by seller, so check first which site sells the show you want.