Online it's exactly 90 days (to the Hour!) based on scheduled departure time.

In the past one could simply call QFF and submit the upgrade request on the day; irrespective of the time.

I presume this is still the case.

Note that time of request is no longer deemed that important. Current anecdotal evidence indicates it's Status of the requester, Class of travel (e.g J>D, L>V) and how long the requester been a member of QFF.