FortiAnalyzer report of time spent per (mobile) application?

On a FortiAnalyzer 5.4.3, I'm trying to generate reports per specific user that can at least estimate their time spent in specific applications.  Since these are mobile apps, their time spent in them during working hours is a useful indicator.

If I look at the logs I can see, for a particular user's mobile device, all the log entries for a particular application.  Each log entry has a duration field (shown as "Duration" in formatted logs) giving the amount of time in seconds for that particular session.  What I don't know is how to sum these up from a given query for that user and application.  I'm also hoping to be able to generate a more general report showing the top mobile applications by amount of time spent in them.

Any pointers on this?  Anybody already have a similar report they'd like to share?

Thanks.