What duration of full body anesthesia will cause damage to the brain? will that decrease your iq? is the brain unrecoverable from the damage?

Odd question: Where did you get the idea that anesthesia causes brain damage? Anesthesia is a controlled sequence of dropping sensation but while preserving/supporting body functions like breathing, oxygen exchange, fluid balance. These do not damage if done properly. If your question is really about coma, the involuntary loss of consciousness,outcome is determined by the cause of the coma.