I'd say it depends heavily on the circumstances.

Remember same-day standby is free on Rapidair, NYC-YYZ and western triangle (YYC/YVR/YEG), which if you're close enough to flight time is effectively SDC.

Otherwise, it's variable. I think the chances are best later in the day when trying to get an earlier flight - seats can open up due to misconnects, IRROPs, etc and if AC can move you to an earlier flight rather than sending empty seats then having to IDB someone on the later flight, it works out welll for them. And best to do it on routes with lots of flights - YYZ-YUL/YYC/YVR/LGA/etc probably a better bet than something like YOW-YYC or YEG-YUL.

Status matters too. As E35K, you're probably less likely to get a free SDC than an SE.