Each user is counted as one APM irrespective of how many times they login/access your content and regardless of how long they have access to it during that month. We count the user as soon as they sign up or login to a Siminar in your plan during that month.The counter resets to 0 at the beginning of each month.
If a user signs-up in January, he is counted as one APM in January; In February, if he does not access your Siminar, he will NOT be counted as APM for February billing. If he accesses the content again March, he will be counted as one APM for March billing.
If your concern is that the participants will keep accessing your content even after they are not supposed to, then you can manually remove their access after they have completed their course or after a time limit. You will be able to see the progress of your participants in the 'Manage' settings of your siminar.