On a normally functioning disposer, the sound changes as the last food is processed. Use your own model's instructions and familiar operating sound rather than a fixed number of seconds taken from another appliance.
What the observation tells you
A new metallic rattle, a stationary hum or water backing up is not a reason to wait indefinitely for completion. Those observations change the task from ordinary use to stopping and assessing a problem. Do not add more food to see whether it clears.
A useful next step
For normal completion, stop the motor and finish the prescribed rinse. Record an unfamiliar persistent sound from a safe distance if service advice is needed.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.