Which brings to mind a couple of questions: How fast can a wizard cast a No-prep spell that is within their Discipline Conviction to cast in one roll?
Going by the single line in the book that provides this sort of guidance, it takes a minute at the very least. Depending on the situation, the narrative tension, and how important the spell is going to be, it could possibly be more like 5-15 minutes.
Can a separator like a chalk Circle serve as a "threshold" to hang a Ward on?
Not that easily, I feel. Harry has been able to ward locations lacking a natural threshold, but it was made clear that he did so by doing a lot of prep work to the site.
On the other hand, Council-level Wizards have reportedly thrown down Ward-level magic on the battlefield, so with enough power, possibly. But for those situations, the community has suggested that Merlin has something equivalent to Sponsored Magic allowing EvoThaum. It could also be that Merlin is actually throwing gigantic Block spells but the fiction is parsing them as Wards for strategic badass purposes.