HannahCatz Hello, my dear friend, apologies for the delay! You are correct regarding the interval's purpose.
The duration of a status effect is determined when being applied to a character. If the status effect is applied via the "Apply Status Effect" function, the "Duration" value of the function determines how long the effect will last. For example, using a value of 3 during a turn-based battle means the effect will last for 3 turns, or 3 seconds if used in real-time.
If the status effect is being applied by a skill, you can specify each duration when setting up the skill (as shown below).
Hopefully that helps! Let me know if I can be of further assistance.