Hi SC!

As was already mentioned it is generally not recommended to report on the transactions, as due to the large amount of data it that is not optimized for reporting - it can affect performance. 

Reporting on the Last Login field of the sys_user would be a better option.
It depends of course on what exactly your reporting requirements are, and how much data you would need. 🙂 Users may also not appreciate it if too much of their specific system usage is being reported on actively. 

If you just want to report on unique users that have logged in during a month you would have to track when a user logs in to be able to report on history.

Otherwise if you want to report on unique users that logged in on the month January- users that have logged in today (March) will not show up anymore.

If you want to track the Last Login time for unique users in a simple way you could define a simple Metric definition on this field on the user table, and report on the metric instances (grouped by time & user).
Maybe even add a condition so that it only generates a metric instance entry per unique month if that matches your requirements.

If you need to track this for a large amount of users in general - it would be recommended to reduce your dataset as much as possible to keep up a good performance. 🙂

Another option could be using Performance Analytics if this is something that you're already using.

Kind regards,
Fernando