The community can best help you if you share the query or at least part of it. Please copy and paste what you can from the actual query.
Without seeing your query, this is hard to say. But I'm betting you're using the
BY clause to sort the average by country. Remove the
BY COUNTRY to get the total average
Either one of the following can help you get the total average duration of all countries:
Remove "by country" in our query
... | stats avg(duration) by country
... | stats avg(duration)
Use the addcoltotals command to view both by-country averages and total averages at the same time
... | stats avg(duration) by country | addcoltotals
Hope this helps. Thanks!