You'll need to create an email action with the "Internal Users" trigger. Next add a conditional to compare against the last visit date substitution. Then configure your email as needed ensuring the to address is the email substitution. Now you can trigger your action using CRON. Please see CB Auto Actions > Config > Internal for the CRON URLs. Please contact your host if you should need help setting up CRON. Aside from that there's no other way to do it.

Please also note by having such a notification you run the risk of your site being flagged as a spammer very easily; be careful about emailing users and ensure the email is meaningful.