Use a Creo ritual (thus at least level 20) of Momentary duration to create something that is permanent: the magic itself only lasts an instant, after which the created thing is non-magical (and cannot be dispelled). The cost is one pawn per magnitude (thus at least 4), as with all other rituals - unless you're a Mercurian
And before you ask, yes, it means it is easier to create something that will last forever than to create something that will last for a Year.
It has to be a ritual, yea... cannot cite a page number, but somewhere it says that a spell cannot last longer than Year duration, unless it is created (that is, Creo as well as vis usage; i.e. Rituals)
If you on the other hand spend a few pawns to get a casting bonus for a Creo spell, it still won't be permanent unless 'twas a Ritual.
Just read every ovvurance of "Ritual" nd you'll pro'lly find it...
Page 112, under Momentary duration: "Ritual Creo spells with Momentary duration create things that last as any other thing of that type. The magic is gone in a moment and so cannot be dispelled. This also applies to ritual healing spells."
and to re-stress - only Momentary Creo Rituals can be worked this way - the spell ends when the Moment passes, and there has to be something left by the spell to continue.
It's not the same as Permanent - the only way to get permanent effects is to (a) create a device with an Always On Effect, or (b) learnt he appropriate Mystery and force a spirit to sustain the spell for you.
Intellego - you got the answer - what more do you want?
Muto - unnatural changes can only be sustained while sustained by continuing magic - when the magic ends, the unnatural chane ends. (All 5e Muto covers Unnatural things, leaving Natural chanegs to Rego) - so
Mom Muto Rituals are over and gone, and ineefectual.
Perdo - things remain destroyed when a spell ends, unless they naturally regenerate/heal/restore (so Mom Ritual perdo is nothing special)
Rego - things stay moved/bent, as Rego only makes natural changes. Changed things are free to change again after any Rego spell ends, and this are free to move - so Mom Rego Rituals are nothing special
And even with Creo - it's only creation of things, and healing (and by extension certain "boost" spells like Increase Characteristic). Not all Creo continues after a spell ends; and after the spell ends the thing is non-magical, too.
THis actually brings up an interesting sub-question.
Perdo and durration.
If I use say, PeTe to make a hole in the ground with Dur Sun, and someone tries to fill that hole while the durration is still active, will the earth be destroyed because the spell is still active, or has the effect been realized, and the durration is meaningless?
Serf's Parma but as far as I could remember; yes - the duration will keep the whole even if you throw more dirt into it - throw branches (He) or corpses (Co) in it it'll fill up nicely. This then leads to another subquestion, what it the spells magnitude was only enought to perdo dirt and you then throw stones into the hole - which would require a bigger magnitude to perdo...?
Out of interest what do people see happening when the spell ends, assuming the stones are not destroyed. The reappearing dirt is not magical in any way but was originally in the place of the stones.
Would the stones be pushed above the dirt as it reforms, slowly and dramatically?
Would the dirt reform around the stones, with the excess dirt piling on top of the original level? It cannot pack itself harder as that is not creo/perdo, though as it is not magical, could this happen naturally?
Would there be some other effect such as an eruption of stones/dirt out of the hole?
Something destroyed by Perdo doesn't reappear when the spell ends (unless you have the Harmless Magic flaw). The duration of the spell merely controls how long it keeps on destroying what comes into the area of effect.
When the spell ends, you would have rocks in a hole, which you can once again fill with dirt.
For someone with Harmless Magic, it would very much be left to the intent of the spell's creator (if applicable) or SG interpretation, on a case by case basis. My interpretation would be that the rocks are simply entombed in the dirt, without bothering with any conservation of mass or volume.