I've been avoiding doing this because I know it's kind of an annoying habit, but what we're basically looking at here is the game's mechanics for Thaumaturgy. The relevant bit is that, when doing a spell, a wizard can safely channel energy based on his or her Conviction stat, which in someone of Harry's ability is going to be 5 or even 6. There's a discipline roll involved, but Conviction sets the limit for the most they can move through them at any one time. The way it works is, power up to their Conviction they can call up without taking any stress, and it's one stress per unit of power beyond that.
Normally, a character can have at most 4 stress boxes; so a wizard with 6 in Conviction could channel up to 10 units of power at once without taking a consequence. Consequences come in four strengths (2,4,6,8), and you can take multiples at a time; in addition, having a 5 or more in conviction nets you an extra consequence at the lowest level. Effectively adding 22 units to that max of 10; so, a wizard of Harry's raw strength can, in one go, take on 32 units of power at once. This is a little less powerful than the game book's write-up of the heart-exploding spell, and that spell is engineered to overcome every possible defense a character could have.
Of course, that's wizards of Harry's level. The newest game book, Paranet Papers, adds a couple powers that can make this even crazier. Mythic Mental Toughness adds 6 stress boxes, so the most now is 38. But at that point, the wizard is all but crippled and a slight breeze will finish him off. One unit higher than that, and they're just plain dead.
Like I said, there are some other wrinkles to the system (that discipline roll, for instance; in order to channel 38 shifts into a spell in one go, you'd have to roll a 38, which is ... unlikely. So, effectively, to cram a spell like that into one shot, a wizard would end up taking two hits bigger than the heart exploding spell. Results are almost certainly not pretty), but that's the best way I can quantify what kind of stress a high-powered wizard can take in channeling energy in one blast.
And that, my friends, is why these guys take their darned time for rituals.
