There are a few things I feel need to be considered with respect to the (very high) suggested number of shifts possible in a thaumaturgy ritual.
From the novel Storm Front, and as mentioned in YS301, The "Heart exploding spell" had a complexity of 36+, and in order for Victor Sells to successfully cast it, he had to do substantial prep work, as well as animal sacrifice and harness energy (inflict Consequences on) his wife and the two Becketts. He also had to take a number of Consequences himself in order to harness the power required, and was also dependent on being able to tap into a thunderstorm.
Harry told Murphy that he himself could summon the power required to cast the spell, but it would kill him to do so, without others helping.
As mentioned in Changes:
The 'larger' version of the heart-exploding spell was the Bloodline Curse that the Duchess Arianna was preparing at Chichen Itza required several days of preparation, with a number of individuals participating (if not hundreds) as well as a number of human sacrifices. Enough for blood to pool in some areas of the pyramid. With the power levels of the principal casters involved (high ranking Red Court nobles/borderline Old Gods), the sheer number of participants available to absorb consequences (hundreds or more), the ready use of human sacrifices (each one providing ~20 shifts) and the use of a Place of Power, this ritual still wasn't ready in a day.
While this isn't the same a Jim or Fred stepping in and saying "a single human wizard can't arrange a spell of ~500 shifts in a single day," it does strongly suggest that one person couldn't do so.
Now, for any spell greater in complexity than the caster's Lore, which means anything using 6+ shifts of power requires the use of Preparation involving Aspects, Scene skipping, Consequences or mini-Scenes where relevant skills are successfully used in a manner which could work towards reaching the complexity of the spell. Lastly, the power of the spell needs to collected and controlled successfully using Conviction and Discipline. The 'slow but safe' method of doing so, would be for the wizard to collect/control a number of shifts equal to or less than the lower of his Conviction or Discipline -2, assuming that would provide at least 1 shift of power is collected per exchange. This would allow a wizard to avoid taking Mental stress from summoning the power, as well as not triggering Backlash or Fallout. If we were to assuming that the wizard is only able to manage 1 shift of power per exchange safely (by either only having Conviction of Average:1 or Discipline of Good:3) then the Heart exploding spell in Storm Front would take 36 exchanges to collect the power needed, unless there were power boosts available from other casters, focus items, etc. Now, while an exchange is a Combat measure of time, a Scene (YS314) could be considered the non-combat equivalent and doesn't have a specifically assigned length of 'story time' it can range from a minutes to a half hour or more. Assuming that each Scene spent summoning power takes ~15 minutes, then a Wizard operating on their own would need 9 hours without interuption, to cast the 'Heart exploding spell'. A significantly more complex spell, that uses 100+ shifts of power, would take the same wizard an entire day (24 hours) or more to summon and cast the power. Again, this would also have to occur without interuption, which means that the wizard would start running into the limits of human endurance. Attempting to push past that would trigger (with me as GM) pretty much automatic Stress on the character, and/or Consequences, and/or free Compels. In the case of a single wizard attempting a 500+ shift spell, that would pretty much be an automatic death sentence for the character in this case, since the character would have been casting (again, no interruptions for minor, unimportant things like food or sleep) for 125 hours/5+ days and automatically the character would have started getting Consequences like
Weary,
Hungry,
Headache, etc which would get Tagged to cause an automatic failure with summonging/controlling the power at some point, with the character then being hit with either Backlash or Fallout, and no character would be able to take 100+ shifts of Backlash or Fallout.