My immigration consultant told me that student is eligible to work full time from the first day after submitting PGWP even if he/she waits for study permit extension. It is just important that you Applied to extend it in timely manner. If your visa was expired and you didn’t apply to extend it, in that case you can’t apply for pgwp, you need to restore your status as a student, apply for pgwp with explanation and in that case you are not allowed to work as long as you wait for pgwp response. On the other hand, there are consultants who think differently… and now, how you suppose to listen??!! I also found one post (4 years ago, bit the law didn’t change since than) where IRCC agent told the same as my imigration consultant..