98% of my tickets are purchased via a 3rd party and I have never had an upgrade not requested. I know it happens, but that isn't the norm in my situation.
That is what Delta.com used to say. It now says time of ticketing. Since the upgrades are supposed to be requested at time of ticketing through all booking channels it should be one in the same now. (I know it doesn't happen all of the time for a variety of reasons).
It might be the case that one has declined an upgrade and then changes one's mind or is traveling with more than two on a PNR so that the upgrade cannot be requested before calling to split the PNR. In such example, the time of upgrade request would be significantly later than time of ticketing.