There is a dependency that the data isn't being aged out before a month.
Check AMP Setup -> General -> Historical Data Retention. There's 2 settings -> 1) client association, 2) inactive client. These settings need to guarantee that a client's data isn't aged out before a month is over - so it depends on client behavior as well.
Once you have that, you run a Client Session report against the group or folder that hosts the IAPs. You can then set the report to start on '1 month ago', end 'now' - and repeat monthly on the 1st of the month. Keep in mind the clients are going to be sourced from the APs of association, not the VC.