New Report wanted. Ave # of emails till action taken by recipients

A thing I have been hoping to discover is how many email touches will it take on average till the recipients take some action, i.e. click a link in the email.  I have searched and seen some ideas on how to accomplish this in Marketo but all of them are far too difficult and complex to implement to be useful.  

Something like that would be useful on deciding how many emails to develop for nurture streams, re-engagement campaigns, etc.