I would consider a difference between a Watching Ward and a spell set to trigger like a booby-trap.
In the case of the Watching Ward, you can set condition which triggers it.The rules are a bit fuzzy about how specific this conditions can be ("simple condition") and require some troop agreements on that wording. But it is quite a powerful effect. It is a kind of smart mine - although it can be used for other purpose than destruction (teleportation, temporary healing, etc.)
Having a spell triggers as soon as something is disrupted - typically a ring - is more akin to a mine that will explode regardless of whom triggered it. The word "collateral damage" is perfectly suited for this kind of spell, therefore, I won't require a ritual. However, if you go with this rule, you have to decide what happen when the ReVi comes to an end: logically, the spell should be cast since it was already casted, but the effect was delayed. If you want it not to be cast at the end of the duration, then probably a Pe requisit (+1 magnitude) should be required.
So I personally found that the ability to have specific, although "simple" condition, to trigger a spell is enough in my campaign to warrant a ritual.
However, having the ability to "suspend" spell in circle as simple trap is a nice Story tool both for players and NPC, thus I would allow it.
Alternatively, you might also to decide to restrict this ability to craft non-ritual, holding spell, to a specific tradition and link it to a minor virtue "Seal's master", descending from Solomon. They used Solomon's teaching to trap Demons in bottle, to derive the ability to trap spell in container (physically) or even simple ring than once open (or broken) releases the spell. This ability allowed them to perform magic discretly amongst mundanes since the spell would have been cast in the quietness of their sanctum, then by opening a pouch, a bottle or a little box, the spell would be released. However, unless ritual magic was used, the power of the spell would slowly wither the container (or the circle) and be released (at the end of the Holding spell duration). So they would be the only one to have access to non-Ritual waiting spell, with duration ranging from diameter to Moon.
Finally, for a mage dabbling in Hermetic Astrology, the new durations brought by Celestial Magic could also be a interesting source for delayed effect: the ReVi holding the spell can be set with duration in minutes, hours, days or Sign and as soon as it wear off, the spell is cast.
Faerie rites allows access to duration which are close to mimicking Watching Ward effect . As I understand the rules (and I might be wrong), these durations are only accessible for Faerie rites (p122 RoP:F, first sentences), not even Faerie Magic done by Merinita's mage. So it will require research and integration into magical theory to have them accessible by hermetic magic, but since another Faerie is probably the closest realm to Magic, it should be doable. Probably a target of 30 research points for "While", "Until", "Not", "If", and another research for "Season", "Aura", "Hidden".
However, I would not assume or grant them at the beginning of a Saga, more set it as a target for mage to access fame and ennemies :mrgreen: (Merinita might not like to see Faerie secret being displayed so blantantly, it kind of weaken their stories - like once you know a magician's trick, the effect is not the same).