What is the difference between length and duration?

Answer 1

'Length; is a measure of DISTANCE.

'Duration' is a measure of TIME.

However, 'length; can easily be, and is often confused, with a period of time.

.e.g. 'How long it takes to do something'.

Answer 2

Length is used to define distance or size whereas duration measures time period.

Actually length IS also used to define time period, as in: how long have you been waiting? So, while length may be used for either, duration is not used for distance.