However, there seems to be a bug allowing you to bypass the cooldown completely. If you cast a spell, then close the spellbook or holy symbol, and then reopen it, the spells are immediately available, regardless of the cooldown.
This bug is specific only to Ravenloft: Strahd’s Possession; the other two games using the Ravenloft engine (Menzoberranzan and Ravenloft: Stone Prophet) have different spellcasting interfaces.
Ravenloft: Stone Prophet actually appears to have the opposite behaviour: if you cast a spell, then quickly close and reopen the spellbook or holy symbol, spells are indefinitely in cooldown until you close the spellbook or holy symbol again. My guess is that this is by design: the game stops keeping track of time when spell selection is active, just like in the inventory.