Actually, you're the first to open the thread.
The decision rests with the Game Master, and until we get a GM's guide for Legend you may rule it either way.
The GM could stipulate a default setting, namely that the enchantment will outlast the creator and only cease when the item bearing the enchantment matrix has finally been destroyed, which could require it to be thrown into a volcano or something.
Other settings, which could be stated at the beginning, could be settings like the following:-
X Wishes: The enchantment does its thing X times, then expires. The enchanted item turns to worthless dross, rots into dirt or crumbles into sand. At the end of a natural termination the magic points return to the caster, no matter where the item is when its job is done.
Expiry Date: The enchantment has a finite lifespan, e.g. "until the Moon reaches its fullness once again thrice" (i.e. three Lunar months measured from the phase of the Moon on the night of the enchantment) and when that lifespan comes to an end the enchantment ends. The enchanted item turns to worthless dross, rots into dirt or crumbles into sand. At the end of a natural termination the magic points return to the caster, no matter where the item is when its job is done.
Until No Longer Needed: The enchantment has been set up to provide a lasting, permanent effect such as protection from something. When that something no longer exists, or when the spell effect is no longer needed, the enchantment expires, as above.
Lifespan Clause: The enchantment lasts for the lifetime of the person who contributed the Magic Points. When the person dies, the enchantment expires, as above.
Perhaps if you include such a termination clause, the entire enchantment would only cost 1 permanent Magic Point no matter how many it would normally cost (but it would still require the full amount of time to enchant as if you were going to spend all those Magic Points anyway). You could stipulate that such a clause adds a -20% penalty to the Sorcery (Grimoire) spell but does not require the allocation of a Manipulation factor to the enchantment.
Thoughts?