According to the IRCC website if I finish my studies earlier than I expect my student permit will expire 90 days after my studies. But when did the 90 days start counting, is it about the convocation, or the last day of my exam?
on the date you get the first notification from your school (such as an email, letter, transcript, etc.) that you completed your program or
when you get your degree, diploma or certificate.
If you can’t prove the date you were first notified by your school, we’ll use the earliest issue date on the document. We may confirm this date with your school.
In that letter will be the date of studies completion so if your last exam was April 15, then your date may be that day or the end of the month.
It says ”or” so depends on the school. Most schools after the completion of your last exam will send you a letter to say you have completed the program so that is the date. So first notification as you may not get your degree for many months.