Actually the 8 means this task runs once per hour at 8 minutes after the hour. So, it will actually run on the next cron job to run at or after 8 minutes after each hour. For example, if your cron runs every 10 minutes, the task may run at 8 after the hour but it may not run until 18 after the hour. You can check the times on this by clicking the log icon for the task to see its run history.
To run every 8 minutes, you need to put */8 instead in the Minutes setting (and leave all the rest as an asterisk *).
For testing, as Shirley says, you would benefit from enabling the Run now button for tasks. How to set that up is explained in the documentation.