If I'm not mistaken, the duration of the spell in rounds is equal to the spell's power, plus 3-4. IMHO, this is still not enough for a large battle. Moreover, the Academy (and Sylvan as well, isn't particularly offensive) is not a castle for blitzkrieg; its units are quite slow and durable (except for the djinns, which stand out from the rest). Besides, the mage can cast phoenixes and elementals, resurrect units, place shields, control enemies, and create phantoms. The battle can drag on significantly. Almost never does a demon, ranger, or knight's spell last until the end of the battle (although even this might be more than enough).