Good question. My experience is Platinums get 2PM, never later, even if the hotel is 50% vacant. That said, I am grateful for the perk. When requesting, I always reiterate Platinum status. 2PM sharp seems to be the standard (regardless of the standard check-out time) across North America for Platinums. This is based on experiences in recent months at:
HI Select Montreal Downtown
CP Montreal Airport
HI Toronto Downtown
HI Mississauga Toronto West
CP Miami Airport
CP Tampa Westshore
HIX Largo-Clearwater
HI Atlanta Downtown
HI Kingston Waterfront
HIX Kingston