Extend Spell

For 50% more spellpoints, a spell's duration will be increased by 100%. This includes the duration of summon spells, however, you may only have one summoned creature at a time.

Tips
Extend is great for granting teammates long buffs, of 10 minutes or more. If you are about to use a shrine and have spellpoints left, consider passing out some extended buffs - but only to players not about to rest themselves, because resting dispels buffs. (That technique is particularly important when you are at a shrine to which you will be unable to return). In general, spellcasters should try to use all their sp on something useful before resting, and extended buffs is one way to do that (aside from repair/heal, of course).