It depends a lot of where you are when you cast the spell, and what is the AI doing (if he can actually get over to your location in 10 turns time) or if he has visibility of the location where you are casting the spell in order to summon units or cast damaging spells etc.

But - IMHO - some changes should be made to Diplomatic / Spell of Making / Epic quest victories, to make AI "aware" of how you try to achieve victory and do something to prevent it.

Example:

AI is aware of your growing alliances, and those who are not in alliance yet will refuse to join it (unless your coin weight and other resources or favors will bribe them into accepting the alliance - and no am not speaking about 100g alliance bought, but something reasonable).

AI is aware you were first to construct the towers needed to cast the spell, and if they are not allies they will try to attack you to prevent you from finishing the cast, maybe something like a general (from those not allied with you) DoW sending everything they have your way.

AI is aware you started the Epic Quest (which is a challenge in and by itself), and should react accordingly, things could spice up, like for example when you get next step to visit X location, you get a message AI Y already been there and is trying to hide the evidence / information, which you would need to either do a quest for them, pay them research knowledge or / and resources, or go to war to obtain it.

Am sure they will tweak those and possibly add more victory conditions (or spice up the existing ones) to make it more challenging