Couple of ways to do this:
Add a new row in Google Sheets (include start time and end time) whenever a new calendar item is created. Once you collate the information inside a Google Sheet, you can calculate the duration of each meeting in a separate cell and then use sum/average hours in a separate column.
you can apply the above functionality inside the Make scenario but it will be more complex. You will probably have to use Math calculation functions and a set variable module to output the average time and store it somewhere.