Stateful equipment use for Red Sign of Shudde M'ell in Arkham Horror
My understanding is that the term bonus in:


  A spell or weapon that gives you a bonus...


is that it refers to any advantages that you may have using the specific card.

Also taken in mind that this card has a hand on it, then it is sure for me that whenever you stop casting it by releasing your hand, then its effect is being cancelled.

I hope my thoughts make sense :)