The Gentle Beast

The Gentle Beast and other ReAn spells have a phrase in the description saying if the animal is "extermely frightened" or similar things happen the spell ends.
I would think the spell ends when the magus loses the concentration or the duration expires. Perdo spells have this effect which is logical in that art but this is Rego.

Because mental effects always include a provision for the spell to be broken by the victim.