This skin currently has no expiry date. However, Epic could remove it at any point, so you should get it as soon as possible. Katalina is free but will come to the Fortnite Item Shop at a later date.
You can get the Felina skin in Fortnite for free by earning a total of 50 Account Levels before December 9, 2024, at 9 AM. You need only 25 Account Levels to get the Felina’s Claw Pickaxe for free.