Yeah - the Rego Vim "Delay an effect" guideline (as discussed above) is your best bet. It pretty much does exactly what you're asking for - however, the write-up of Watching Ward (which is the implementation of the effect) does specifically say that it is a ritual due to the potentially-infinite duration of the effect. - that is, watching ward lasts potentially forever, until the Trigger for the effect is enacted.
However, that implies that non-infinite durations are possible. As Eric Tyrrell mentions, you could simply have a "Duration: Sun" delay effect - at which point the spell releases at sunrise or sundown. I would argue that you could create a non-ritual Circle/Ring ward, ("Lasts until the circle is broken"). Both the "potentially infinite duration" issue, as well as "breaking a ward", are both well-understood effects already built into Hermetic Theory. However, I would qualify that with "it's not just crossing the boundary of the ward - it's literally breaking it" - so you'd likely have to design the circle so that it was easy to break as your target crosses over it. This may or may not be easy, depending on how your troupe defines "breaking a circle".
Perhaps a more flexible combination is the Concentration duration on the "Delay Effect" - which means you can link it up to a "maintain the demanding spell (ReVi)" - so you can set the actual duration to anything you have a Demanding Spell variant for. (Arc/Day/Month).
Before Rival Magics came out, I usually interpreted that non-Ritual Delay Effects could have triggers - but if the trigger didn't occur, they would fade naturally at the end of their normal duration, and both the ward and the spell inside the ward would simply not go off. I also determined that a ward without an additional Intelligo spell built into it simply had the senses that the magi who cast it did. So you could design a default Watching Ward to go off if someone entered the room - but if the magi was blind, then you couldn't use Sight to define it - you'd have to use Touch or smell or Hearing, or something like that.
EDIT - actually, now that I think about it...a non-trigger watching ward really isn't a watching ward. It's not triggering on anything, so much as the "delay effect" part is merely expiring. After which the spell goes off as normal.