Rarely, very rarely. Having the guarantee to successfully cast the spell, but cause some consequences in form of Complications, that makes Sorcery too “reliable” for my taste of Conan-like Sword&Sorcery. Sorcery should be a reckless gamble, nothing you will ever be able to rely upon to get a desired effect.
And Complications are just that, they can make things more complicated, but must not devalue, not take away the success of the action the occured on. So the success of a spell casting attempt will not be diminished or turned into a failure on a Cast for Consequence, even if you get a lot of Complications.
That does not fit for my view on Sorcery in a Conan setting.
I do use this method for NPCs, though, as they only exist to make things more interesting and challenging for the players, so they need to succeed in their evil deeds - and the Complications could be paid off with Doom or the NPC visibly corrupts and withers before the very eyes of the PCs, all fitting for the mood I like to create in a Conan game.
Rarely. It is a rare case that any PC sorcerer utterly fails at casting a spell. It might be that the spell casting does not create sufficient Momentum to get the desired effect, but that a spell fails completely is a rare thing. - Sorcerous offerings, high Willpower, Momentum, Doom, Fortune - all that helps making the usually not that frequent spell casting actions successful - to a degree.
And then, most PC sorcerers who actually know one or even a few spells are built for being successful at sorcery - to the detriment of nearly all other skills. So they have high TN and high Resolve, which are essential for a sorcerer character.
“Dabblers” in sorcery might fail more often, but then they are only dabblers - and for most (if not all of my) players dabbling in such a risky thing as Sorcery is not worth it at all.
So, you get rather competent sorcerers who rarely fail at their spell casting using the normal rules for skill tests, but sometimes they don’t get the sufficient amount of Momentum for their spell to get the desired effect or to activate all the options they were looking for - and sometimes (not that often, actually) they roll Complications. Sometimes with bad luck on the dice a lot of complications even on a successful casting of a spell. And that is, how Sorcery in Conan should be from my viewpoint.