Thaumaturgy is generally described in the rulebooks as taking minutes to cast and the general rule of thumb is not to let players cast it in combat situations, that being the province of evocation and/or enchanted items instead.

On one hand I agree with this approach, however on the other hand it is, in my opinion, somewhat inconsistent with the source material.

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For example Harry manages to cast very simple thaumaturgy spell in a few moments behind some cover in order to provide magical protection to Murphy in a combat scene.

It is also inconsistent with the actual game mechanics which state that if the caster’s Lore is greater than the spells complexity then they can cast it with what is on hand. Given that, if the caster can channel sufficient power in one exchange and control it, then they can cast thaumaturgy at the speed of evocation.

This is potentially a problem as it down plays the importance of evocation and sponsored magic (which allows thaumaturgy at the speed of evocation).

On the other hand, typically thaumaturgy spells will be less powerful than those of an evoker as they are limited by three skills (being Lore (= complexity), Conviction (= amount of power able to be channelled) and Discipline (=control)), rather than two for evocation (Conviction and Discipline). On the other hand they can be cast for one less stress (i.e. so long as the power to be channelled doesn’t exceed Conviction + Refinement/Focus bonuses) and, in general, last longer.

Given this, I think it is necessary to slow thaumaturgy in combat down, without removing it entirely. I also think the ritualised nature needs to be played up. Given the above, an in discussion with the primary thaumaturgist in our group we came up with the following time frame for casting thaumaturgy spells in combat:

Requirement for casting thaumaturgy in combat: Complexity of spell must be less than caster’s Lore.
Exchange 1: Form and Empower Circle to contain spell;
Exchange 2: Gather Ingredients for Spell (which must be on hand, see requirement for casting);
Exchange 3: Channels Power;
(Exchange 4+: Channels more Power if necessary).

This process can be sped up by “rushing” the process, e.g.:
Exchange 1: Form and Empower Circle to contain spell;
Exchange 2: Gather Ingredients for Spell as a supplementary action and Channel Power at -1 to Discipline;
(Exchange 3+: Channels more Power if necessary).

Or by having assistance with the creation of the Circle, i.e.:
Exchange 1: The caster empowers a pre-formed circle (e.g. drawn by another character) and gather’s ingredients as a supplementary action;
Exchange 2: Channels Power;
(Exchange 3+: Channels more Power if necessary).

Thus it takes a minimum of two actions and either a Discipline roll at -1 by the caster, or a second character’s action (to draw the circle) to cast the spell. Furthermore, we ruled that if a character chooses to defend themselves, they must start the ritual again, or if they choose not to defend themselves they must make a Discipline roll with a difficulty equal to the shifts of damage inflicted to not lose the spell.

This still means that Sponsored Magic is useful re Thaumaturgy at the Speed of Evocation as it enables you to use Thaumaturgy in a single exchange thereby forgoing the risk of having your spell disrupted or making yourself a target.