The best indicator that a customer has finished one of your products is when a certificate for it is issued. This is a great opportunity to upsell them whether that be to other courses or to extra services you have. One way to do it is by using the "Certificates Issued" trigger to start nurturing them.

By clicking on Add filter you can keep it only for a specific template or use one of your custom fields to make it more specific.
 

Click on Save Trigger to set it. From then on it's up to you to do interesting things with your workflow! Will you use it to send an email campaign and invite them to your next conference or will you leverage your audience to make more sales of your awesome products? We're sure whatever you decide this will help you achieve it.