You have three hours to show up to the gate ready to depart. If you're R08, and they call you right at 0800 you have to return the call within 15 minutes. Once you've returned the call you're notified. If that happens at 815, then you must arrive at the aircraft ready to depart by 1115. You don't need to push by 1115. There's no requirement anywhere that says you must have the plane ready. Only the pilot must be ready to depart. Show up fit for duty and have all the required items with you. If you think the plane needs to push within three hours of notification please educate yourself, call your rep.