I did this on a SQL server not that long ago. It was a pretty straight forward task back then.
I have been thinking how this could be done in Qlik ETL.
There are many ways this could be done. One thing you could do, is to create all the minutes in an interval, and then do an innerjoined intervalmatch. But i think this could be a heavy task to do, if you have many rows to work with.
Attached is a solution, where the idea is to calculate the time per day and then sum this up per FromTime/ToTime.
My solution is based on an excel spreadsheet where you define the workhours per day. I will suggest that you autogenerate this instead.