where can I see which licenses have been assigned to which user at what time?

This may help to some extent. As a System administrator for Teams/Enterprise, you can download detailed reports on how end users are working with corporate assets, such as folders, files, and libraries. These reports are called content logs. 

See here also:

https://helpx.adobe.com/enterprise/using/users.html#Exportuserdata

And check out the related Forums discussion here:

Tracking Usage of Licenses

Hope that helps.