You can setup the sending frequency in wysija’s settings page.
Wysija > Settings > Sending Method
The minimum sending frequency you can set is of 15 minutes. On the other hand you need to be sure that either your site has enough traffic(one page view each 15 minutes), or you have a cron setup pointing at your site’s cron url http://yoursite.com/wp-cron.php
Then you just need to let wysija take care of the rest.