Craft TaskService Overview/Cron Job
Craft's tasks are meant for long-running PHP requests and don't have to execute immediately.  By default when you create a new task, it is queued in the database in the craft_tasks table and it takes a control panel request to trigger it to run, so with that method, there's no guaranteed way to have it run every 15 minutes.

However, if you set the runTasksAutomatically config setting to false, then you can setup an cron job that runs at any interval you wish.  You'd point it to a controller action on your plugin and manually trigger the task that way.