There's no required time one needs to be on screen to be deemed a supporting role. I could see why you would argue something like that about a leading role, though I'm sure such a requirement doesn't officially exist, but not about the supporting rorle. Speaking of which, the Academy has often made very arbitrary decisions who to nominate for Leading Role and who for Supporting Role, often based on whether someone is a big star at the time or not. Some of the silliest examples: Kevin Spacey playing a "supporting role" (?!) in Usual Suspects; Emma Thompson and Kate Winslet, both leads in Sense and Sensibility, but one gets nominated for leading role and the other for supporting role (guess which one was which); Travolta being nominated for a "leading role" and Samuel L. Jackson for a "supporting role" in Pulp Fiction. Either they were both "leads" or they were both "supporting", since it was an ensamble movie and neither of the roles was bigger than the other.
^ I remember readin' that the Academy deemed Travolta the lead of the two because he had more screentime than Jackson. For all of Jackson's screentime, he was with Travolta, but Jackson isn't there for all of Travolta's.
Aren't the studios the ones who decide which category they are going to push for a nomination? If they think it's easier to get a nomination and win for supporting actor, they lobby the academy members for that nomination, even when the role is a lead? Sally Field's Mary Todd Lincoln is the female lead in Lincoln. But an older actress is going to have a harder time than a young one. So when the studio did those ads and mailings they push for supporting actress. Isn't this sort of thing pretty much what happens?
It's not the studio, the performer makes that call (the studio obviously has input, since they like to coordinate campaigns). Christoph Waltz switched abruptly from campaigning in lead to supporting this year (which worked out nicely for him), dropping in on his costars who'd been there the whole time. Sally Field is the most prominent actress in Lincoln, but she is not an "actress in a leading role". The First Lady is clearly a supporting character. There have been any number of performances that blur the line between the two categories and where placement is arbitrary (Anthony Hopkins was thought to be a supporting actor in The Silence of the Lambs, but he asserted that he wanted to be considered a lead). Helen Hunt would be a better instance of a lead actress positioning herself as supporting this year for strategic reasons.
I doubt anything will ever top the silliness of Jamie Fox's Supporting Actor nomination for Collateral. He was in all but one or two brief scenes!
Uh, no. Look it up, Foxx was absolutely the lead, had rather more screen time than Cruise thanks to the opening sequence, and was still nominated for Supporting.
I seem to recall that some actress (whose name escapes me) won Best Supporting Actress for one scene in Network.
^ Beatrice Straight. Ned Beatty was nominated for best supportin' actor for his role in Network, which amounted to two or three scenes, but he only had dialogue in one.
I think nothing tops Kevin Spacey being considered an actor in a supporting role for Usual Suspects - because, if he's not the lead in that movie, who the heck is?!