Learn French: L'elfe se demande quand lancer le sort. - The elf wonders when to cast the spell.

This is an indirect question using an infinitive. When the person wondering is the same person who will do the action, you can put a question word like quand directly in front of the infinitive lancer. It works exactly like the English phrase "when to cast".

Why do we use se demande instead of just demande?

The verb demander means "to ask" someone a question. When you use it reflexively as se demander, it literally means "to ask oneself". This is the standard French way to express the English idea of "to wonder".

I thought lancer meant to throw. Is it the standard word for casting magic?

Yes! While lancer usually means "to throw" or "to launch", it is also the exact verb used in French for casting a spell (lancer un sort). Think of it as throwing magic at a target.

Could I use this infinitive structure if the elf was wondering when a wizard would cast the spell?

No, you can only use the question word plus infinitive pattern when the subject of both actions is the same person. If the elf is wondering about someone else's action, you have to use a full clause with a conjugated verb, like L'elfe se demande quand le sorcier va lancer le sort.