Much like all processes in nature itself, folk magic spells have a limited shelf life. The energy behind them slowly drains out over time. Nothing is eternal.

This is especially true of magic in shintô.

omamori are only good for a year. hitogata are a single use and then they're ritually destroyed. ofuda are replaced annually. shide are renewed every six months. shimenawa are replaced when they begin to look weathered.

Even the main shrine to amaterasu at ise is rebuilt every twenty years.

There are also yearly matsuri of renewal, where the original reason behind the festival is reiterated and strengthened.

So expect your spells to not to last forever. Your mileage may vary, but when their energy runs out, they will stop being effective.