Is there a canon answer to what happens in the case a mage creates in a faerie regionne, a circular ward against the local type of faeries, with high enough level and Penetration to stop the top faerie?
What happens to the Glamour in the warded area? How soon?
Well, I think you've trapped part of the Faerie's glamour in your circle until such a time that the Faerie dissolves it.
I believe the aura a Faerie generates by extending their glamour works in much the same way that a sword that belongs to a faerie knight. It's incidentally part of them, but isn't a force they "generate".
It's part of them that exists independently of the body they have.
If you cast a Faerie ward around the sword of a Faerie knight, the sword would not stop being a sword. It would remain until the Faerie knight needed that part of their glamour for their story.
Now, one could argue that the knight would need to break the ward to reclaim that part of themselves, but until then it's a bit independent of them- like Faerie gold that remains until its role has been played.
So, the Faerie aura wouldn't decrease, because what you've done is trapped that part of the Faerie's self inside of the ward. The Faerie would have to show up and find a way to break the ward, since they cannot cross it to affect that part of their glamour.
You may want to look at the Devil's Lane in Mythic Locations. The faeries in the lane sell magic items, pure glamour, with a duration of seven years IIRC.
I have no reason to think that there is one universal answer, so that all glamour have the same duration, but at least it is canon input ...
Think of it like toons in Roger rabbit "I can only do it when it's funny" the glamor will last as long as it suits the narrative.
I think it would be trapped inside the ward, the same as if you cast a circular ward against humans around you and your shield grog, your shield grog doesn't banishes or gets kick out of the ward. So going back to the example above, I don't think a faerie knight's sword inside the ward wouldn't be able to dissolve it as long as the ward lasts.
I'm not sure if the knight would just shrug and manifest another sword out of his own glamour, probably. Depending on his understanding of what's happening, he probably wouldn't think of it (our maybe it just would play along, because stories).
If you cant glamour to fade then you probably need Perdo, not Rego.
The poetic answer is until the tides have turned and the moon has withered and the last petal of the rose has fallen.
My initial gut instinct is that there's fey stuff which is projected into the world, and if you cut that stuff off from the will which is making it then it will fail and fade. But if something fey is trapped within the circle, then it woud persist. So the existential question here is whether this regionne is projected from some central core or if it's pervasive throughout its entirety.
But perhaps not. Perhaps the glamour is part of the fey, and so trapping part of its glamour within such a circle would be like trapping a person's hand in a glass jar. That part of the fey is confined, but (unlike the hand) not dying. This could anger the fey. Or amuse it.