In shinto, generally speaking, magic comes with a "best if used by date". When sufficient time has passed the spell's origin impetus dissipates and it comes to a halt, or worst explosively terminates.

The physical expression of a magic spell has to be either periodically recharged, or disposed of.

omamori and ofuda are good for about a year and then should be ritually deactivated and destroyed.

hitogata are used for purification and then thrown into either fire or water.

shimenawa and shide are replaced when the elements and time begin to degrade them.

There are rituals for retiring worn out tools, utensils, and toys. They are thanked for their years of service and given funeral rites.

Even some shrine buildings are rebuilt on a cyclic basis. The main building at ise shrine is replaced every 20 years or so. The old shrine is dismantled and the reusable timber is donated to other shrines that need repair.

So don't expect your magic spells to run forever. They will eventually "give up the ghost".