The spellcaster is able to manipulate the duration, range, damage and chance of success of his spells. The total MP used in a spell cannot exceed his rank though there are a few exceptions to this rule, e.g. Wall of Magic. The number of MP expended is equal to the spell level plus additional MP spent on spell manipulation, if any.
This only applies to durational spells. For spells with Spell Expiry Rolls (SER), each additional MP doubles the frequency of SER. E.g. 1 additional MP means SER is made every 2 CR, 2 additional MP means SER is made every 4 CR, etc.
For spells with duration listed as a fixed amount of time, each additional MP doubles the duration. E.g. For a spell having a base Duration of 10 min, 1 additional MP increases Duration to 20 min, 2 additional MP increases Duration to 40 min, etc.
As a rule of thumb, the duration may not be increased beyond one day.