How long is a mission is task driven and airframe dependent.
If the task is to operate at short range and there are lots of aircraft then 24 hour coverage could be maintained with each aircraft flying for 6 hours with 5 hours on task.
A similar task time a 1,000 mile from base requires a flight time of 10 hours and more aircraft because of the greater transit times.
If extra crew were carried and flight refuelling to extend endurance, then you could use fewer aircraft.