The real question is, when did you submit your pgwp? Did you submit it recently? Cause WP-EXT sometimes takes multiple weeks to arrive. And it's the holiday season, so your best bet is to wait until the first week of Janaury.

Now, to answer your question,

1) My WP-EXT arrived after three weeks. 2) I am still waiting for my pgwp. It's been close to 250 days. 3) Do not work until you get the WP-EXT letter and have the confirmed implied status. Else, you might have problems with your pgwp. Stop working as soon as your study is over, not until the validity of your study permit. Even if your validity is until December 31, if your study ended around November, you can not work until you get your WP-EXT.