The completion right now is iimplemented based on the View event. It should be based on events captured when the session ends.
There too many criterias for an activity though BBB to be considered as completed. It may be time of permanence during the session, it may be participation, it may be through a point system that would require the Teacher to summit, it may be specific excercises or anything else.
As neither BBB nor Moodle should force a criteria, as a first approach the completion will be based on time spent in the session.
BBB has the ability to calculate the time a user has spend in the session and convert it to a percentage. That result should be sent to Moodle trhough the plugin and this even would be triggering the completion