There is some information available in the Change Log section for each process. The change log will give you the date the process was initially created and then also the date it was published. This can also be accessed and downloaded for multiple processes/groups using the Process Change Log report which can be exported to an XLS file to be analysed in Excel or PowerBI.

There doesn't appear to be any further information in the documentation about the change log although it is briefly mentioned in the ‘Archive, Delete or Restore a process’ documentation.
 https://help.nintex.com/en-US/promapp/Processes/ArchiveProcess.htm.

Additionally, the Processes table in the reporting API provides some more granular data, i.e. the time and date the process was created and also the ‘StateChangeDate’ which appears to be around the date & time the process was most recently published.

There is some more information on the reporting API here https://help.nintex.com/en-US/promapp/ReportingAPI/GettingStarted.htm

Hope this helps,

Eddie