There's probably a convoluted way of doing it
It may not match up exactly, but you could create a "Work Days within Last 30 Full Days" segment by doing something like:
With some custom Date Ranges, you could do a series of these for "Last 30 Days", "Last 60-31 Days", "Last 90-61 Days", etc (maybe with a better naming convention)
If you mean the results of these segments, it should get you close to your Average Workweek Month?
If you don't want these to be rolling based on "Today"... you could use set Date Ranges, breaking the year into a series of 30ish days, excluding the Weekends.