I don't remember ever overflowing SoA despite playing wizards regularly.
On the other hand, I never cast it enough to last for the whole game.
Generally it's mana dependent. Considering you're a mist elf, either you're already 25+ in mana department or will soon be.
I believe that it's best to just cast it occasionally and wait for it to run off. You retain more control and there are no surprised.
I can tell you that as archmage with 99 mana, casting it 10 times lasted me for the rest of the game, but that was usually done at the point where not much game was left to play.
I don't think there was any testing done specifically to find out if and when SoA overflows.
I suppose you're kind of exploring an unknown land with this
"Hell is empty and all the devils are here."