Duration is how long something lasts, from beginning to end. A duration might be long, such as the duration of a lecture series, or short, as the duration of a party.
The noun duration has come to mean the length of time one thing takes to be completed. The duration of something might be known or not — in past times, the unknown length of time the current war would last was called "the duration. "Musical notes, such as whole notes, half notes, and quarter notes, have different set durations, or lengths of time for which they are held. These varying durations, when put together, create the rhythm of the music.