I think there are too possibilities here - a reasonable amount of time (<5 hrs) and a long amount of time (>5 hrs). I personally think reasonable would mean 4, but obviously if someone's battery charges in 4.5 hours it's still pretty "reasonable" also, so we'll make the boundary 5 for simplicity.
I noticed also that my MBA seems to charge rather quickly when turned on whereas when it's off it takes 8 hours. So I guess you should choose the time based on how you prefer to charge the battery (for me it is when the computer is off, overnight).
I noticed also that my MBA seems to charge rather quickly when turned on whereas when it's off it takes 8 hours. So I guess you should choose the time based on how you prefer to charge the battery (for me it is when the computer is off, overnight).