From Dungeons of Dredmor Wiki

Cooldown is a timer associated with spells that do not require mana to use. After such a spell has been cast, the Cooldown comes into effect and the spell cannot be used again until the timer has elapsed.

Each spell has their own individual Cooldown length, scaled to the power of the spell to prevent overuse. These lengths, signified by an integer, represent the number of turns that must pass before the spell can be used once more.

Currently, the size of the UI prevents Cooldown lengths over 99 from being displayed on the spell's icon. However, the Cooldown is still operating as normal regardless.