I feel that folks may be inadvertently forgetting that the GM is generally setting the DCs for the Lore Declarations. The rules mention (at least for Divination effects like Harry's tracking spell) to just let certain low-power Thaumaturgy rituals work, without requiring the "minigame" that is the spell construct and empowering phase.
I really want to avoid this as it makes communication between the player and the GM much harder. The whole point of well written rules is so that the GM and the player can both read the rules and then both have the same understanding of what something can do.
So, the rules should address things like:
How powerful a ritual do you need to do X?
What kind of characters can be expected to be able to do X?
How do they get from the base complexity to the complexity of the power they want?
How long does it take to do the complexity preparations?
How long does it take to do the ritual?
And if you can't answer these questions with the rules, then it's always a question of "GM, may I?" and this sort of thing tends to produce adversity between the players and the GM, even when it's not really the GM's intention to screw the players over.
Now, I'm also not a big fan of the whole mini-game aspect of thaumaturgy. Too many hacker experiences from cyberpunk games. What I'd prefer is a system where the player can do the 'story' of them spell for themselves in a few minutes, then just present that to the table at large. Having the sub-system require a lot of interactive GM time is not a great idea, imho.
so, to recap how I answered the questions:
How powerful a ritual do you need to do X?(see examples in the book)
But the rough maximum power level to be handled by the system 50 which includes thing like summoning scary demons (refresh 15+), transforming people into other (scary stuff) stuff, killing a bunch of people at once, blowing up a small office building, or making a minor item of power (hexenwulf belt, for example).
What kind of characters can be expected to be able to do X?Anyone who can get control 5 in some sort of thaumaturgy is ready for at least medium power rituals (power 20 or so). Being able to do higher power rituals takes a bit more work, but is doable by 8 refresh wizard who specializes in big rituals. Being better makes things faster, though.
Anyone who can't muster control of at least 5 is mostly relegated to the kiddie pool, not because they couldn't do a bigger ritual, but because trying a higher power ritual is a good way to get yourself blown up.
How do they get from the base complexity to the complexity of the power they want?The majority of the difference between the rituals complexity and base complexity are bridged by using skill declarations. Skipping scenes and invoking aspects are for special occasions.
But to keep things 'interesting' you can't just use the same skill over and over again for your declarations.
How long does it take to do the complexity preparations?From no time at all to 'several days'. Being better speeds up your prep time a lot. A starting wizard should be able to do medium complexity (~20) thaumaturgy in several hours.
How long does it take to do the ritual?Usually between 30 minutes and several hours. Faster ritual times are generally for dangerous circumstances.
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And most importantly, the player has a pretty good idea of whether they can do effect X in time Y, and once they tell the GM that they are going to do it, the player can take a few minutes to do the rolls and write the declarations, and then just come back to the table and tell everyone how it went. The GM can be doing another scene, or something.