duration is the length of time that the valves are open
you need to have the valves open long enough to fill the cylinder.
think of filling a jar under a tap
when its full some flows back towards the top or splashes back
now put the lid on this jar,shake it and walk into another room,open the jar and pour out the water in another sink
how fast you fill the jar depends on the neck size or opening size(valve size in an engine)the water pressure from the tap and tap size (intake runner size)
lid on as soon as the jar is as full as possible without spilling
empty jar by turning upside down with lid off(think of a long neck pop bottle Vs a short neck pop bottle for exhaust)put lid back on as fast as possible so you can go back and refill at tap.
any wasted time and spillage is cam/head/carb/header inefficiency