So, the limit is how much lore the caster can put together? That's not something you can just keep pumping in to the spell at intervals?
The way Thaumaturgy works is:
The most complexity you can put into a spell without further preparation is your Lore (plus whatever specializations or focus items for that type of spell you have). If this number is lower than the target complexity of the spell, you have to take actions to build it up, at a rate of +2 for each declaration or maneuver done. So someone with a Lore of 4 who wants to cast a 10-shift spell needs to take three such actions.
There's really no hard limit here to how much complexity you can build up--but unless your GM is
seriously lowballing your competition, you're going to run into difficulty here because eventually someone should catch on to what you're doing and try to stop you. There's also availability of proper spell components to consider against your available resources.
Once you've met the complexity and you've got the power and the sympathetic focus, you can start casting. Once you start casting, the spell's "locked" in--you can't add more complexity, and you can't pause to do something else. Each round, you can channel power up to your conviction rating without stress (or with one point of stress for each shift above it), and control that bit of power with Discipline. If you blow the discipline roll, either you let
all the spell energy go as fallout and the spell fails, or you take
all of the shifts put into the spell by that point as backlash and keep going. This means that anything above 10 shifts or so is
very dangerous to the caster and everyone around him if there's any chance of a screw-up, especially if they're normally a low-powered character.
Again, there's no hard limit, but obviously the higher complexity spells are going to take longer to cast safely, and the more often you're rolling, the more likely that you're going to roll a big -4 and blow the whole thing. Plus, this is another portion where your enemies can interrupt--while a caster is busy rolling out discipline, they're a sitting duck.
So in summary, grossly huge thaumaturgy is only possible when you either don't have serious opposition looking to stop you and you have a huge span of uninterrupted time to yourself, or you've got a legion of people to protect you while you cast.