Calculating then sorting by Highest to Lowest Average for Case Data in Excel

Hi!

I'm hoping someone may be able to assist with something I am trying to achieve in excel with regards to calculating the average case resolution time on case data for a set of customers, so I can then sort my spreadsheet from highest to lowest to identify which customer's cases are taking the longest to resolve. 

I've attached a very simplified example of what the spreadsheet looks like - but the actual data I'm working with contains several hundred customers with over 4000 cases (rows) meaning I need to find a way to automate this through a formula of some sort. 

Any help would be appreciated 🙂