Pretty much anything is possible with Thaumaturgy—if you can imagine it, the right ritual can accomplish it. The limitation is always going to be how complex the ritual is, and the resulting time it takes to pull that off. The important thing is to figure out what the spell is meant to do first, and then find the rules that best fit it. (Everything in DFRPG goes that way: you don't find rules to do what you want, you figure out what makes sense to try within the world of the game, and then the rules will happily help you figure out how to represent that mechanically.)
The closest bit of mechanics to what you're describing is a ward (YS p. 276)—instead of a ward on your home, you're sort of placing a ward on yourself personally. Wards in the book have a lot of specific special effects that go with them, but we're going to ignore most of them, because of what I said above: you pick the effect first, and then you find the rules to implement it; so we're not going to use the special effects that usually accompany place-based wards (like rebounding attacks) or the limitations they have (like being fixed in one location), because that's not part of the spell you've already come up with. We're just going to use the basic mechanics of wards to give us a mechanical framework for implementing your contingency-based shield, since it's convenient and doing this won't break the game by any means. In more general terms, we're looking at a "lasting change" type of Thaumaturgical effect, since it effectively changes you, and lasts longer than evocations do.
So to contingency-protect you, we just need a very basic "ward". We're just using the ward as a container to hold and release a linked spell, so we don't need the full complexity that comes with a normal ward. The strength of the ward can therefore be Mediocre (+0) since we want it to just crumple and release the linked spell when you succumb to an attack. That gives a starting complexity of zero, a nice place to begin!
Duration is covered on page 265, and is pretty much up to you. (The eroding effect of sunrise on magic is already taken into account by Thaumaturgy's duration mechanics.) You start from the duration of a scene, and then work your way up the time chart (p. 315) until you get to the duration you like. Let's say for this example that you want the contingency to last for "a few days"—you've just uncovered some trouble that needs investigating, you're not sure what you're heading into, but a short couple-days ward will probably do until you know more. A scene is about "15 minutes", and you go up six steps to find "a few days", so that's +6 complexity for 6 total so far. So far so good.
Now we have to add the shield that triggers. A triggered spell in a ward adds complexity equal to the evocation's normal power. Let's say we want a shield that does a Great (+4) block, to give you some great protection after you're hit without making the ritual too hard to pull off on short notice. But wait, we also want the shield to last longer than one burst of protection, so we need to add more power to increase its duration—for "a couple of rounds" we only need to add 1, since the first exchange is free and maintaining a block evocation for extra time costs 1 per extra exchange. So that gives us the stored shield's power of 5, which brings our running complexity total up to 11.
There you have it: a contingency shield that holds for a few days and can pop up for two exchanges after you've suffered a consequence is probably something like a complexity 11 ritual. That's something you might be able to pull off with little notice if you have Legendary Lore, but more likely your Lore is far less and you'll need to do some preparation. If your Lore is 4, that's 7 shifts of complexity you need to cover with preparation—that's something you can probably scrape together with a trip to your local weird-ingredients shop (Make Declaration preparation option, +2 shifts on success), using a relevant aspect on an object of power you've squirreled away and one on yourself relevant to your emotional motives, (Invoke Aspects prep option, +2 shifts × 2), plus taking the time to secure and prepare your ritual space and do the ritual itself (Skip a Scene preparation option, +1 shift).
With longer contingency durations or a better shield (or an armour version of the shield, which doubles its base cost), you can see how the complexity could shoot up and require quite a bit more preparation. On the other hand if you're heading out the door and directly into trouble, you can see how you could knock down the length of the contingency and get it down into a ritual that's simple enough to do in a hurry.
You ask about limits. Thaumaturgy doesn't really like limits, and doesn't impose any on you. The real limitations are that you have to actually do the preparation—you can't just ask the GM to let you skip to the end and just have the effect, you have to actually play the preparation (at least as a montage). If you're making a declaration, you have roll for that declaration and risk that e.g. the trip to the magic shop being futile and having to do plan B to build up your shifts of power. If you're invoking aspects, you have to set those up beforehand, conveniently have a perfect aspect already, or recruit the help of someone/thing that does. If you're Accepting or inflicting consequences, that takes time and has, er, consequences.
So you have to choose your prep options, and then detail what exactly they are, and play them out to see what happens. Meanwhile, time is passing and the enemy is still active, and you can be caught up in trouble while you're still preparing. The more complex the ritual, the more ways your preparation can go wrong or be interrupted, the more ways you may need to find worse alternatives to build up the shifts of power necessary.
You can have as many of these contingencies on you as you like, but setting them up takes time, resources, and risk; and the world does not stand still even for wizards. If the demon is going to be summoned tonight and you have lots to do before you're ready to face it, you pretty much have one shot at getting the ritual done, assuming nothing goes wrong first. If you're aiming to set up layers and layers of defences, that simply gives reality more opportunities to mess with and disappoint you. These can be mitigated, but that's where the limitations lie: these things take effort and time.
So don't think in terms of limits—think in terms of interference and Murphy's Law preventing you from "stocking up" on Thaumaturgy-created contingency spells.
If you're still grappling with what Thaumaturgy can do for you and how to pull it off, I highly recommend Rick Neal's article "Thaumaturgy, or How to Break the Rules", the third part in his series on magic in Dresden Files RPG. He's got some very smart insights into the system, derived from extensive play.