Average Email response time (From all incoming emails)

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Hey guys, 

I think I have been going through most topics regarding this topic. However, I am still struggling to find an answer. 

What I am looing for is: 
Average response time on all incoming emails:   XX Hour XZ Minutes  XY Seconds 

Iv'e seen some examples, but from my understanding they are not truly what I am looking for (some examples just take an average on the first reply etc) 

How can I fix this?
How can I find this data that will be used in a general KPI report?

Currently using: avg(if(inc_performance.intv_type=1|inc_performance.intv_type=2,inc_performance.rel_time))/60