如何在Android中正确安排意向服务

I'm working on app in which I want to schedule service in a interval of 6 hours. I'm calling this method from main activity. When this activity open then it call this method and hits the service. I don't want to execute it whenever this method executes. After first exceution of this service it should execute after 6 hours or so not app open. Is there any flag or something I need to do set to do that.I'm working on app in which I want to schedule