In Harmony, the length of each scene is determined by the amount of frames it contains. You can calculate the amount of frames your scene requires based on the amount of time it should span over with the formula, in which l is the scene's length, s is the amount of seconds your scene requires, and fps is the scene's frame rate.
l = s × fps
Likewise, a scene's length in seconds can be calculated with the reverse formula:
s = l ÷ fps
By default, scenes created in Harmony are 60 frames in length. At 24 frames per second, this makes a scene of 2½ seconds in length. You should set your scene's length based on how much time you plan for it to require after creating your scene so you can plan your animation based on the scene's length. However, it is possible to adjust your scene's length at any time. You can also insert or remove frames at any point in your scene.