One of my creatures got frozen, and spent 19 turns remaining frozen (I started just defending to see how many turns it would last, eventually the boss died, so no idea when frozen would have worn off). From reading the in-game text about the debuff, creatures should have a 25% chance to break out which doubles every turn they do not. I’d guess this means they should always get unfrozen on their 4th turn maximum? That did not happen here.
Battle history: ***** Your Flesh Abomination casts Firestorm. *****Gore Abomination gained 853 - Pastebin.com (Flesh Abomination is the one that got stuck frozen)