A quick way could be if you click on your cog and check for workflow history, it will show history of all the workflows thats run on your site with time etc.
Obviously there are also Nintex workflow reports (if you have subscribed to enterprise license) where you can get quite a bit of WF reporting.
Sorry wasn't aware that you are on o365. I think you should target your question to "Nintex for O365" group rather than "Nintex for SharePoint". There would be quite a few people who would respond to your question then.