You can relist the item as soon as the auction has ended, but
you must relist within 90 days if you want a credit for it.
If you are needing to relist because the original winner did not
pay you, you should raise an "unpaid item dispute" against them so
that you can get your Final Value Fee credited back to you. The
whole unpaid item process currently takes a minimum of eight days
and you should not relist before the dispute has been closed.