Let's set the needed parameters

In the backend of you site go to System -> Scheduled Task

Create a New task
Select the task Type Delete Trashed
Set the the interval
Set the execution rule

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Details

Task Plugin: Delete Trashed

With this plugin you can schedule to run a delete of trashed items..

Install

Download the Plugin zip file to your local machine.
From the backend of your Joomla site select Extensions → Install/Uninstall.
Click the Browse button and select the plugin package on your local machine.
Click the Upload File & Install button.
Note that plugins must be enabled before they will work.

Settings

Once the plugin is installed and enabled you need to go System Dashboard → Scheduled task

Create a new Task
Select the Task type
Set the desired interval
Save

Step by step guide