Days Due vs Days needed - what if you when you add up delays each month it is way more than needed?

Days Due vs Days needed - what if you when you add up delays each month it is way more than needed?  -  So do you have to give an extension for way more days than necessary just because there was fat in schedule, or they are able to condense the work?  If they will be done with a job in 20 days, do you have to give an extension for 60 days because that is what all of the delays added to? - obviously the "effect" was less than the raw numbers.