how many?

so how many times must you answer the same question before you can change the answer. for instance if you tell someone they can have something. and then they ask if they can have it. you say yes. then they ask again. not more than 10 minutes later. again you say yes. when they ask again, is it permissible at that point to say "no, no you may not have it". how many times must you answer the same question before you can change your answer?