Most spells are assumed to be either instantaneous or permanent, unless the player describes them otherwise. Other than setting the initial duration, the mage has no control over duration once the spell has been cast. Cancelling a spell, even one of your own, requires a new spell. Loss of such trappings may cause penalties, at the GMs whim. These rules assume 50-200 point campaigns.
Elemental mages are a bit slower and more power-hungry than normal mages, but much more flexible.