from Saunders’ The Basics of Magic

Whether performed by magical creatures with innate magic or humans who must harness magic from outside sources, all magic spells require equal energy to perform them (i.e. payment in kind) as well as maintain them over time.

Once spells are cast, most must be periodically recharged (i.e. energy added) to continue using them, especially if they are frequently activated. For instance, communication mirrors must be recharged after each use, often by leaving them in the sunlight (although that can vary depending on how they were enchanted.)

However, some magic spells do not require recharging because they had a renewable power source set when casting them. For instance, contraceptive charms are typically powered by the wearer’s pumping blood, so these spells don’t require recharging, just renewing once a year.

In addition to recharging, most magic spells must be renewed (i.e. periodically recast) to maintain them. How often spells must be renewed is set when first casting the spell, although the typical time if one year. If spells are heavily used, they will need to be renewed more frequently. For instance, if communication mirrors are used nearly every day, they often must be renewed twice a year rather than annually like normal.

Powerful spells may not require renewing if both a renewable power source and automatic renewal were set when they were cast. However, only Rhiannon descendants or magical creatures can successfully do this, and the longer powerful spells last, the more likely they can become corrupted and fail. This is why the elves have lore masters check the efficacy of the Walle every six years.