Hi,
The issue here is indeed that the timeline is empty - Studio::EventDescription::getLength specifically returns the length of the timeline, but the event pictured has no timeline, and your asset is being played using an Action Sheet. While there are ways to get the length of the asset currently playing on an Aseets sheet, or to receive a callback when an instrument in an event has stopped playing, they are both more code-heavy than simply moving your single asset to a timeline, so I would recommend doing that if possible.
If using a timeline doesn’t fit for your use-case (e.g. if you’re planing on playing multiple dialogue assets using a single event), can you please provide me with a little more detail on your implementation of this audio behavior in FMOD Studio?