You could just run several smart lists to start with for a general idea.
Grab your revenue stage filter or Lead Status filter and select your stage or status value of say Raw/MQL etc.
Then drag n' drop the Was Sent Email filter, is Any, in the last 30 days with a constraint of min Number of times of 1
This will give you all leads at that stage who was sent an email at least once in the last 30 days. You can then start adding more minimum days and/or increase the date range.