What you need to do I write a log message at the start of the event and write a log message at the end of the event.Make sure you write the date and the time the event started.
Once you have this information, you can use microflow to calculate the amount of hours between the two dates (hoursBetween (date1, date2), which you can calulcate after commit of the second log event.
Make sure your Errorhandling is correct, so that the log message are also printed when something goes wrong in the scheduled event
At the start and at the end of the event microflow, you could save the system time to a variable or log object and then compare these values. The difference in milliseconds could be easily calculated in this way.