Ring duration is sort of 'outside' time. It can last 10 seconds or 10 thousand years, however long the ring lasts. Which is an encouragement to make longlasting rings out of permanent materials and then hide them.
Ring duration spells can easily go down without warning or at inconvenient moments, when the ring is broken, unlike Moon or Year durations.
i was thinking about ring duration spells the other day. Does the ring need to be visible? for instance, my magus creates a 30 foot steel ring. he drops it in a equally sized moat around a shed. then casts his ring spell on the ring.
Could he then bury the ring and still have its effects last until the ring is broken?
This would create a durable ring that is not only hard to damage, but you can't see the ring either.
Some years ago now, I was playing in a saga in which demons and ghosts kept cropping up to cause mayhem. Having suffered considerable aggravation casting circular wards against demons and/or spirits, only to have something break the ring, I was eventually compelled to have my character invent the following (imaginatively named) spell, revised for 5th edition:
Pythagoras' Circle CrIg 10
R: Touch, D: Mom, T: Ind
The magus touches a point upon the ground, and a mathematically perfect circle of up to 15 paces in diameter is magically scorched into the grass, floorboards, carpet or whatever. Note that the magus need not trace the circle, only touch one point of its circumference, and that the spell may place the magus inside or outside the circle. The fire is hot enough to scorch even stone or damp ground. Unless it touches something really flammable like a pool of oil, however, it will not ignite anything, only scorch it.
Base 5 (Create a fire in an unnatural shape i.e a ring.), +1 Touch.
It's a handy way to make a fairly indelible ring without carrying a tin of whitewash around. Does that work under the new rules? I was concerned it might have to be R: Voice or T: Room, but I thank that can be circunvented by starting from a base of 5 to create the fire in a ring. Obviously the spell is useless if standing in ankle-deep water, or on one of the middle ages' many asbestos floors. An ideal spell for the lazy infernalist, demon hunter or spirit master, it's best not cast in somebody else's home without seeking permission first, or at least offering to buy new rugs afterwards.
I realise this doesn't answer the questions posed; they just reawakened a fond memory.
Ok, if ring can last essentially forever, why would you not build a really big ring for casting a spell like Aegis of the Hearth?
The concentration task for drawing the ring could be sped up by having drawn the ring in advance over a couple of days and then casting the ritual to activate the ring.
The concentration element can be handled with a fairly simple CrMe spell that boosts the concentration ( I would compare this to the effect of CrCo for boosting recovery rolls ) and then you can get essentially a cheap and endless Aegis of the Hearth spell instead of one that has to be cast every year and costs pawns of vis to constantly renew.
If you just want an excuse for saying, as a Storyteller, that it can't be done, then I'd just rule that the Aegis is a pre-Hermetic spell, and that modified versions can't be invented. Since Rego Vim spells to provide magic resistance are now impossible under 5th edition rules, that's a fairly comprehensive rationale.
The Aegis of the Hearth ritual as it stands is a much an element of the setting, and a dramatic tool as anything else, so I'd be inclined to argue against letting the players tinker with it.
That, of course, is a bit of a non-answer. Mechanically, at least if you were to allow the players to use invent Rego Vim spells based upon the same criteria as the Aegis, as they could in previous editions, there's nothing to stop them from doing so. If they were wise, of course, they'd devote plenty of thought to (ahem) keeping their ring safe from tampering.
All this talk of rings, however, is beginning to enter the dangerous territory of double entendre...
With duration Ring, at least in fiction, the ring is broken when the caster crosses it. One could easily rule this in order to limit the power of the ring somewhat. This tends to be the case, even when the ring is etched into a stone floor, or made of a metallic circle. It's the caster interacting with their own magic field that makes the ring fall, not the destruction of the ring itself. This allows for such things as permanently inscribed summoning circles that aren't "always on."
Whilst it's a decent idea to curb misuse of the Ring duration, and an atmospheric one, I don't think it's really supported by the rules. It would also sadly preclude the creation of ward-type spells to contain creatures within them, in the classic "demon imprisoned in pentagram" way, since the caster would be unlikely to wish to be stuck in there with whatever he'd just entrapped. Oh well...
Breaking a ring need not require the ring to be physically destroyed. A chalk line over an iron ring is a break. Likewise, the caster crossing the boundary also could be considered breaking the ring. This doesn't mean that the caster must stay in the circle, it just means that whichever side he is standing on when the spell is cast, he stays or the spell is broken.
It's very much in paradigm for a simple, symbolic act to "break" a ring. The demon trapped in the stone inlaid pentagram only needs the clumsy apprentice to spill an opaque substance across the line to "break" the prison.
Don't forget, there are two ways to end a spell with ring duration:
a) until the target of the spell moves outside the ring
b) until the ring is physically broken.
I'd think that if the caster was supposed to stay inside the ring as well this would be explicitly stated.
And for just putting a line of chalk over the ring: I would not consider something like a leave on the line as a break. I always understood this as: "A part of the ring has to be erased." There are various examples for this interpretation in literature as well (e.g. in Goethe's "Faust" - a play I would always recommend to potential Ars Magica SGs as inspiration - Mephistopheles is trapped in the protagonist's study by a pentagram. But being a powerfull devil he commands the rats of the building to destroy the design. They gnaw until a bit is missing, they do not just put their tails across the line.)
A well prepared covenant probably has some circles engraved in its dungeon to keep dangerous creatures permanently inside without being forced to burn vis. (cf. the "Captive Magi" thread)
I would have to agree that physically breaking the ring is more than a natural event and requires some actual direct act.
There might even be a relationship between magi using chiseled stonework to avoid problems like ink stains or rats gnawing on woodwork.
It also brings up the question of whether Stonehenge and similar stone circles are large rings for casting spells or maybe have spells already in effect that the foolish might disturb releasing the original warded contents ( maybe a trapped earth spirit or demon ? )