I don’t think it’ll be easy (and I haven’t done this myself), but I think it’s possible using the REST API for two things

1. Users
2. Posts

You also can do exports of these, but you’d need to do “All time” exports and refresh them regularly. 

With a BI tool (Tableau, Power BI, etc), you should be able to link those two data sources by userId. Then, you could create a table that pulls in 

• UserId / username
• Registration Date
• Date of first Time Stamp (from the Posts data)

You should be able to also then calculate the difference between those two date and average/median it out. 

Could you do the same thing in Excel/Google Sheets? Probably, but there’s a lot more risk of messing things up in the many steps along the way. From my experience, BI tools will make it a lot easier to 1.) model and 2.) refresh.

If you don’t have the expertise and/or tools needed to do that, I recommend finding a colleague who does.

Pulling @Daniele Cmty in here . I believe he has a similar metric he reports on. 

You are welcome @Daniele Cmty 

Thanks for your input @DannyPancratz - I believe that this could be the way to go, let’s see if I manage to do it 

Hi there, Thank you for the mention!

This is actually not a metric we report on. I’m sorry!
 

I would follow @DannyPancratz’ approach here. 
We use Tableau so this is actually a good inspiration for our tables :)