I used to be a hard-liner when it came to thaumaturgy, thinking even the most basic spells take too much preparation time to use in a conflict. However, I was persuaded in these forums that it's possible to be a little flexible on this, simply because it just isn't a realistic option in a fight. You can do far more with evocation, or with almost any other power, in a conflict than you can with a ritual. It's slow and clunky and inefficient, only small spells can be done (due to being limited by Lore) and even then it generally takes more than one exchange to cast and it's easy for the enemy to interrupt the basic preparations made.
It's also hard to abuse; many spell effects require special components and casting the ritual mid-conflict wouldn't change that. Targeting an enemy might require an appropriate ingredient like blood, hair etc even if they do have a high enough lore.
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dlw32 - it states on page 252 of Your Story
"Performing maneuvers is a little trickier than attacking and blocking. By default, pulling off most maneuvers requires 3 shifts of power, but if the target has an appropriate resisting skill rated higher than Good (+3), that skill total determines the required number of shifts."