duration vs span

duration and span both are nouns.

Word	Noun	Adjective	Verb	Adverb
duration	Yes	No	No	No
span	Yes	No	Yes	No

As nouns, span is a hyponym of duration; that is, span is a word with a more specific, narrower meaning than duration:

Other hyponyms of duration include clocking, longueur, residence time, stint, stretch, time scale, note value, time value, value, rule.

duration (noun)	span (noun)
continuance in time	the act of sitting or standing astride
the property of enduring or continuing in time	a structure that allows people or vehicles to cross an obstacle such as a river or canal or railway etc.
the period of time during which something continues	the distance or interval between two points
	a unit of length based on the width of the expanded human hand (usually taken as 9 inches)
	two items of the same kind
	the complete duration of something

duration (verb)	span (verb)
	to cover or extend over an area or time period