Let's say I cast a non-ritual Creo Herbam spell to create, say, a stick. The spell is still active: the Duration hasn't run out. Someone comes along and casts Perdo Herbam on the stick. The stick is obviously destroyed.
Does it stay destroyed? Or, since the magic is still active, does the stick re-form the next round? Is that covered in a book somewhere?
EDIT: an implication from the Fast-Cast rules is that their spell would have to be a higher level than mine to destroy the stick and not have it re-form. Not sure I am reading that right, though.
It stays destroyed, just the same as if by mundane means someone had taken the stick and put in a fire - it is destroyed. The ashes may not disappear until the spell does, but destruction is destruction whether it's by magic or mundane means.
There are Perdo spells which do not destroy their target forever, like e. g. every Perdo spell cast by a magus with HoH:MC p.87 Harmless Magic.
There are also Creo spells - like many of those with D: Concentration, e. g. ArM5 p.121 Deluge of Rushing and Dashing - giving their caster control over the effect while she is within (ArM5 p.111 Ranges) range.
So an answer to that question will always be case-specific.
In general, a creo spell creates something and then sustains it. It (typically) doesn't do continual creation. There are lots of perdo spells that do continual destruction. As a general rule destruction does not need to be sustained.
I have to agree with OneShot on this one: "it depends".
Assuming that the stick reforms does feel "weird" - after all a mundane stick would stay destroyed.
But if a magus creates fear in a target via Creo Mentem for D:Moon, and a second magus removes the fear via Perdo Mentem for D:Sun, many (most?) would agree that when the second spell expires the first reasserts itself.
So...
I look at it this way-
if you create a plant with CrHe for a duration moon, and another magus causes it to be sick with a PeHe with a duration of a day, then when the Pe spell runs out the plant will recover, because that is what plants do if they are still alive.
A stick is not alive. A mind is alive. Living things recover from Pe spells that do not kill them, and the spells upon them recover with the living thing.
I am not convinced.
Consider the following scenario. A maga makes an unliving item very hot with a D:Moon CrIg spell. Another magus makes the item very cold with a more powerful D:Sun PeIg spell, that overwhelms the first spell but expires before it. When the second spell expires what happens?
If I understand you correctly, you are saying that the item goes back to "room" temperature. I would say, instead, that it becomes very hot again until the first spell expires.
fire is a different animal, so to speak in that creo ignem specifically maintains heat in the absence of fuel- it would be more like if the second mage had cooled a fire without putting it out, just that the manifest fire does not actually exist.
That one's actually covered pretty well by the Fast-Cast guidelines. If the PeIg spell is less than 1/2 the level of the CrIg, nothing happens; if the PeIg spell is between 1/2 a level and the level of the CrIg, the item returns to being very hot once the PeIg expires; if the PeIg spell is greater than the level of the CrIg, the item goes to room temperature because that actually breaks the CrIg spell.
My understanding is that the casting of non ritual creo magic give enough power to create the subject of the spell and sustain it for the duration but not enough to recreate it should destruction occurs. I don't think there is a direct ruling on it though, the best insight should be roundabout discussion on the subject like in HoH Mystery Cults in the Criamon chapter (the harmony vs strife concept).