When does the 40 minutes limit start?

Hi I am using zoom to give an english class to one student. Both of us are using free plan so we have 40 minutes time limit. So I was wondering when does the 40 minutes time limit start? I usually open the zoom meeting like 5 minutes prior before the student enters the meeting. Does the time limit start based on when I opened the meeting or when the participant enters the meeting?

I am really curious about it but it's so hard to find exact detailed explanation on this issue.

Thank you