The rules define an exchange as being as little as 30 seconds, or as much as a few minutes. Thaumaturgy's big limitation is that the simplest spells take minutes to cast. My own POV on this is that Thaumaturgy exchanges always fall into the 'a few minutes' length, while combat defaults to 30 seconds. So while characters may have 'a couple of exchanges to react' that may only amount to a minute, while casting that ward may take 3 or 4 minutes - or you can rush through it in 2 (but that would obviously put the aspect of 'rushing it' on the spellcasting attempt
). Obviously if the time the PCs have to prepare is 4 or 5 minutes then there's no real problem with them throwing up a simple ward, but the question then becomes why are they being given so much time and why would they spend it casting a ward when they could simply leave the area by other means?
The rulebook also states under the Optional rule that allows you to reduce complexity by sacrificing effect time:
The GM should also make it clear how short is too short—if the effect is shorter than a scene, why isn’t the wizard doing it with evocation?Being able to perform a ritual with no prep isn't the same thing as being able to do it quickly. It just means that you have the materials with you to perform the ritual. Given the shortage of time you've really only got aspects to rely on if you need to boost the power. And there's always the chance that you'll need those fate points to re-roll the casting rolls.
Personally I never allow a Thaumaturgy spell (even a minor one) to be performed in under 4 exchanges in combat (so nothing less than about 2 minutes). It's not just the actual channelling of power you're dealing with, you need to take the time to create the circle and set up any ritual elements. Once you're performing the ritual you can't really take any other actions without breaking the ritual, and I'd be inclined to say that any dodges that require a great deal of movement will break the ritual as well.
Plus, Wards aren't portable. So no running away. And you've just shortened your ward's duration too... If you'd simply done an Evocation block you could either have the block against damage from the enemy while you run away (giving you shield or armour), or you could block their movement to give you a headstart. By comparison the ward comes in a shoddy second unless you absolutely have to remain where you are, and if you have to remain where you are then shortening the Ward duration isn't really an option because you'd want it to be in effect as long as possible, and you'll need to make it strong too because otherwise the enemy will simply break it down. Bear in mind they don't have to break it down, with a good enough roll, or a creative enough approach they could attack through it.
