Instructors are responsible for the endorsements they signoff. But then again, everytime you give someone instruction for anything, say a rental checkout, its an endorsement. If that person has an accident, well, they have a private license at least and need to explain. Instruction must be given but the instructor cannot be in that plane forever after that. The safety of the flight is up to the "Pilot in Command." Im doing a complex and high-performance signoff now. The student has about 10hrs of high performance time with me and about 5 hours of complex time. Not giving him the endorsement yet. No minimum time required for these but its my name in his book and hes not yet proficient enough. INSTRUCTORS MUST JUST ALWAYS USE THERE HEADS!!!!!! Someone gave a signoff without flying with that person......IDIOT!!!!!!!!!