My personal experience:
my medicals exams request was sent on my bday, Sep 9th, did them right away on Sep 11th, that same weekend I sent the request for my police certificate (trying to be proactive because I heard the next thing will be the police certificate)
September 17th: Canadian Embassy in Mexico received the fingerprints and photos to start the process for my police certificate
October 4Th: the request for the police certificate was sent from the Canadian Embassy in Mexico to the mexican authorities.
October 16th: I received the official request from ircc to submit the mexican police certificate, deadline: December 16th. (They gave me two months)
November: I requested an update to the Canadian Embassy in Canada, they mentioned mexican authorities can take from 30-60 business days (not includes the weekends !!!!)
December 4th: I asked again as I have not received any update and the deadline is approaching, the Canadian Embassy in Mexico was not able to give me any update, but told me to request an extension period of time to deliver the document.
December 9th: (1week before my deadline) I sent a case specify inquiry to ircc explaining my situation and asking for more time,!I received an answer two days later telling me they sent my request to the proper department (nothing on regards of my extension deadline)
December 16th: the deadline day, I called ircc in sidney to request for more time and apologize for not been able to deliver on time. Ircc mentioned they already got the mexican police certificate in Dec 11th (this means they received it but did not update the application)
Ps: I proactively requested my police certificate one month before ircc requested it and I barely made it on time !
ps. : a few friends living in Mexico did it on their own in a PGR and they got it in around 15 days!
advice: if you can do it in Mexico, do it ! Saves you lots of time !!