My character has Conviction 3, Discipline 3, Lore 3. So she can typically expect to do this in about five exchanges. To be safe and because she's in no hurry, she's opting to raise two-points of energy across seven exchanges and one point at the very end.
If you're not in a hurry, I'd actually recommend using one shift of power per exchange. Using two is mildly risky with a Discipline of 3, all it takes is one -2 roll you're not prepared for*. Dropping that to a single shift of power means you'd need to roll a -3 to fail, which is much less likely. (You'll get a -3 or worse ~6.2% of the time and a -2 or worse ~18.5% of the time.)
*When you are in a hurry, set up at least one aspect related to controlling power and save it for boosting a low roll.
My question is this: How long does it take to perform an exchange in an unhurried thaumaturgical ritual?
From other games, I generally accept that a combat-exchange will take about three to six seconds... It feels wrong for a thaumaturgical exchange to take the same length of time as a combat-exchange. I mean a 15-shift ritual shouldn't take only 45-seconds to a minute and a half, should it?
I don't differentiate much between thaumaturgy and evocation exchanges. After all, Sponsored Magic allows thaumaturgy at combat speed.
What really makes the difference is the preparation. Have you ever tried drawing a relatively even circle several feet across? Not easy to do quickly. Consider all the aspects being set up and how they're set up - runes, circles, candles, incense, stone arrangements, other items, mental prep, etc. All this takes time. But keep the ritual below Lore so you don't need the aspects and you can cast as quickly as you can control the power - potentially in seconds.
