Insert the next recurring charge date in custom table when user purchase a product?
I have done something similar although I don't think it's the best way.

I created a rule that is triggered with the event I wanted and then added an action which was custom php. Then use drupal db api to make the update or insert in the table.

https://drupal.org/project/rules
https://api.drupal.org/api/drupal/includes%21database%21database.inc/function/db_query/7