You can download attendance reports from your Zoom sessions, and see when attendees logged in and out, and how long they stayed.  

• Log in to Zoom
• Select ‘Reports’ on the left navigation bar, then select ‘Usage.’  
• Set the duration of the time frame for your meetings.  This will give you a list of all meetings in that time frame.
• The ‘Participants’ column at the far right shows the number of participants.  Click on that number.
• A new window will pop up – ‘Meeting Participants.’
• Select ‘Export with meeting data’  then ‘Export’
• A .csv file will download with the attendees information.