Very interesting design indeed. I like the ability entitlement too (Impervious).
As it is a common, we speak for an effect of 5 turns per hour or 60 turns per 12 hours or 120 turns per 24 hours.
If you redact the cost, it is 35 turns per 12 hours.
It is not worthwhile despite the nice design. Perhaps we can tweak rarity, casting cost and time to reward turns to make it have more usability than it has now. Like Rare / No stacking/ 6 hours enchantment / 1 turn per 10 mins 3 turns to cast.
Or we could make 4 rarities out of this card. The better the rarity, the better the reward. The 4th rarity (Legendary) could be really powerful for 1 time usage and could be incorporated into the Witch Hut.