How to dynamically calculate averages for time values?

Hey there, 

I am having an issue to dynamically calculate averages for time value, these are my columns: 

as I firgured out, there is no way to do it with the duration column, as it allows to perform 'count' and 'count (distinct)'. My idea is to change time period dynamically (e.g. Last Month, Last Week). The problem is there are many duration rows for a single website, that's why I need to calculate an average. 

Looking forward to your answers.