if I cast Magic Circle into an Ioun Stone of Reserve, do I spend the 1 minute casting time when I put it into the item, when I release the spell from the item, or both?
You spend the one minute casting time, store the spell, and when released it goes into effect instantly. I get this impression by the wording "treated as if you cast the spell" which would seem to indicate that the spell has already *been* cast, so it would then take effect. It's a slippery one and I can see why there might be other interpretations.
TL;DR: You must spend the full minute casting the spell
Relevant text:
"While this stone orbits your head, you can castany spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell. The spell cast from the stone is no longer stored in it, freeing up space."
You are using the item to cast a spell. The DMG clarifies how this works:
Some magic items allow the user to cast a spell from the item, often by expending charges from it. The spell is cast at the lowest possible spell and caster level, doesn’t expend any of the user’s spell slots, and requires no components unless the item’s description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration. Many items, such as potions, bypass the casting of a spell and confer the spell’s effects with their usual duration. Certain items make exceptions to these rules, changing the casting time, duration, or other parts of a spell.
While the spell does override some of the rules, such as slot level, it doesn't make an exception to the casting time of the spell. That means you must use the normal casting time of 1 minute for magic circle.
if I cast Magic Circle into an Ioun Stone of Reserve, do I spend the 1 minute casting time when I put it into the item, when I release the spell from the item, or both?
You spend the one minute casting time, store the spell, and when released it goes into effect instantly. I get this impression by the wording "treated as if you cast the spell" which would seem to indicate that the spell has already *been* cast, so it would then take effect. It's a slippery one and I can see why there might be other interpretations.
<Insert clever signature here>
TL;DR: You must spend the full minute casting the spell
Relevant text:
You are using the item to cast a spell. The DMG clarifies how this works:
While the spell does override some of the rules, such as slot level, it doesn't make an exception to the casting time of the spell. That means you must use the normal casting time of 1 minute for magic circle.
Okay, so 1 minute on the way out, that makes sense. Thank you!