MusicDuration

MusicDuration[duration]
MusicDuration[name]
MusicDuration[properties]

represents a symbolic musical duration.

MusicDuration[1/4]
MusicDuration["Eighth"]

Useful properties include "Duration", "Beats", "BeatDuration", "Multiplier", "RealBeats" and "RealDuration".

duration = MusicDuration["Quarter"];
duration["Duration"]
duration["PropertyAssociation"]

Numerical musical durations use whole-note units: 1/4 is a quarter note and 1/8 is an eighth note. WLJS converts these durations to seconds using the active MusicTempo.

Sound[{
  MusicNote["C4", MusicDuration["Quarter"]],
  MusicRest[MusicDuration["Eighth"]],
  MusicNote["E4", MusicDuration["Eighth"]]
}]

See the Wolfram Language reference for beat and real-time duration forms.