So, we were discussing magic items and we got to an idea:
a tiny scabbard that when you touch your sword to it, the sword shrinks to fit inside.
Now I thought that you would need to use a concentration effect:
Base 4: Shrink an item by factor of 100, +2 mags to affect metal items, +1 mag for Touch, Concentration +1, (+5 lvs for item maintains concentration), 24 uses/day - in case you need to remove and re-sheath it (+5 levels), Environmental trigger (+3 lvs) = Lv 33 effect.
But my player countered quite reasonably that it could be a momentary effect if she gave the item unlimited uses:
Base 4, Touch +1, Momentary +0, Metal +2, 10 levels for unlimited uses: 25
The environmental trigger is because they don't want the sword turning back to full size at sunset.
But I have a gut feeling that enchantment effects aren't meant to work like that.
Your player might have to explain just how the item is activated. By touching the scabbard to a metal item? Or how else?
As the activation of the momentary effect must not need any activity by the scabbard's wearer, it might cause a lot of mishaps and mischief.
I don't think you'd run into issues at sunrise/-set. I do however believe that the enviromental trigger (as previous effect runs out) is required to stop the sword from bouncing to full size every 1-2 seconds.
My biggest counter-argument is, that this has (often) a smaller level requirement than regular continuous effects, but can be turned on and off at will (although with potential Magic Resistance issues).
Concentration + Item holds it: 2 Magnitudes + Number of uses
Sun + 2 uses + enviromental trigger: 2.8 Magnitudes.
Momentary + Unlimited use + Enviromental trigger: 2.4 Magnitudes
That is correct, just touching the scabbard to a metal item. She planned to essentially wear it as a hair pin, so there shouldn't normally be too much metal around that area.
We did thankfully notice that the momentary effect version wouldn't work in this particular case as it would inevitably bounce off of the mage's parma or require them to keep their parma constantly supressed.
The most compelling reason is that a wielder has to activate the trigger. This can be avoided by an environmental trigger (+3).
IMO a momentary effect does not have a duration, and unlimited does not allow an infinite number of castings. I would argue that only one casting per round is allowed, and the momentary effect does not last the round. However, related questions have been debated inconclusively in the past.
Iirc, technically Momentary has a single turn duration. So, up to six seconds (ofc, it could be shorter depending on the effect).
The tricky question here... its which Environmental trigger the player wants? Because if the object is recasting the spell nonstop. IMO, it must be a trigger thats not constant but its happening nonstop (i will explain what i mean with an example: the object being under light its not a good constant trigger, since it will only trigger the first time that enters under the light, but since the condition isn't changing anymore, the trigger isnt triggering again).
I have not seen any RAW statement to such effect.
Furthermore, canon spell descriptions say otherwise. PeCo to make a wound just happens; it takes a literal instant. Playing combat by the book, the wound penalty applies at the next initiative even if the round is not over.
There is a reason why it is called momentary and not turn.
And yet, MuVi spells that are Momentary, can affect a spell cast on the next round. We have canon spells that have one or the other interpretation, and both are valid, depending on spell.
Some of them stretch out in time just for the fluff Look at "Curse of the Rooten Wood" for example. It could do the effect immediately? Sure! It does? No. Why? Rule of cool
If rule of cool is a valid factor, then somebody can do a spell that makes you blink invisible for half second... and there is no reason to tell that its not valid...
But again, the real question here is... which is that Environmental trigger that activates the spell at least once each X but at the same time nonstop? Imho thats what makes the object into an impossible task ^^! (maybe you could make a piercing with the trigger "when somebody exale into it", but... its usefulness will be limited since it would need range and penetration to affect other items into you... and dont dare to gasp or you can end with a surprise xD)
Having read what's put here I think the issue makes sense with the "trigger" being the problem.
"A trigger can involve a command word or phrase, moving the item in a specific way (for example, waving or pointing a wand), a stance to be adopted or anything physical that you can imagine. Most enchanted items cannot read thoughts, so he trigger action must by physical, not mental. By default, the trigger action must be performed by someone holding the item, although intention does not matter. Wands with offensive powers usually have very specific triggers, to make sure that they do not go off by accident, other items trigger whenever they are put on."
So putting a ring on your finger or putting night soil in a chamber pot is an action that can serve as a trigger. However something being inside the magic item already isn't a trigger as nothing is actively changing.
Maybe, but the effect is not stable for any duration. It takes a duration to build up, but once the effect is achieved, the spell ends. I do not care if you let it take a round to shrink the sword, but once it is shrunk, the effect ends, and it starts to unshrink. Happy to let the unshrinking last a round too, but it would not solve the problem of fitting in the scabbard.
A momentary spell lasts for a moment. It is in the name!
So how long is a moment?
It is an indeterminate but short amount of time.
In Ars Magica terms it would be anywhere between a fraction of a second and up to a couple of rounds.
There are several canon spells with Momentary duration where the spell description makes it obvious that the spell must be active for at least a short amount of time - i.e. the duration cannot be immediate for those spells.
And I'd say that with Silent Shout, the sent message must be stable while it lasts or it would be uninteligible.
Tbh, at this point I don't feel that we are talking about rules anymore, but about playstyles.
Imho, for such trivial things and in a game like Ars Magica, rule of cool should trump to an overly literal reading of rules.
So, with such a small thing that doesn't break the game at all, and having to choose between having a long discussion with the player because we have different opinions or letting him have his cool sword that it remains miniaturized for a second before it recovers the normal size. I would choose the later option.
Also, following the sword example, I would play it like that the sword then needs a turn before its usable as such after its being unsheathed (earlier than that, it could be used as a dagger... or a knive... or a pin... I don't know the exact size that the player wants to give it when its sheathed xD). I'd say that this would lead to a much more interesting outcome that just barring it because "I say so".
My beef here is not with how long a moment is but that there already are ways to create duration effects in the rule system and this just tries to bypass them, making them irrelevant, just to create the same effect at a lower level.
Your player should create a tiny sword instead and have the scabbard make it large for a diameter when drawn
Look at "Curse of the Rooten Wood" for example. It could do the effect immediately? Sure! It does? No. Why? Rule of cool 
