You can export worklogs to Excel with the Better Excel Plugin, and report on the exported data.
The particular use case you mentioned time-spent per user per issue per month is available with one of the default templates, called worklog-report.xlsx, that creates a pivot table similar to this:
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs