From the RAW there are a few ways to represent the greater endurance we see in the later books. One of them is an increased number of spellcasting consequences. This would be the wizard who draws himself dry so much he starts to get an increased ability to do so.
I forget, were there stunts that gave more mental consequences for just that? Because offhand, I can only think of increasing Mental consequences through the skill increases, and most games you're not going to get a wizard with 7 in Conviction.
Another way is actually to segment conflicts. When Changes came out a bunch of people asked Fred "Wait, how the hell were Harry and the other wizards able to cast so much in such a short time?" His response was that there were actually several scenes in that conflict, separated by short pauses for Harry to catch his breath, so to speak.
I was thinking about that, but those scenes struck me more as an example of the discussion Billy and Dresden have in the margins in regard to stress going away after a scene, unless circumstances have it where you're going right from one fight to another. The big fight in Changes struck me more as that than actually giving Harry breaks.
I know neither of those are necessarily the kind of endurance you're looking at, but I would look at increasing the number of spells that they are capable of casting through the expenditure of resources (I.E. Refresh) rather than attempting to make all casters capable of greater endurance by changing the core rules.
I think it still works out that way--you're still bound by your effective Conviction and Discipline, after all, so if you still want any power in your prolonged spell, you've got to pay for it with higher initial stress and less duration. Someone with higher Conviction (i.e., higher spent refresh) is going to have significantly increased endurance for equal spell power than someone with lower Conviction.
The way I look at it, if you have two casters who're pumping out Weapon:4 spells round after round, someone with an effective conviction of 8 ought to be able to do it easier than someone with an effective conviction of 4. The less experienced/powerful wizard ought to be huffing and puffing after four rounds of that, while the big badass wizard should still be good to go.
With the RAW, they'd take exactly the same stress if the newbie is casting at normal power and the badass is casting Weapon:1 spells, and that just seems off to me.