Judi Dench got an supportin' Oscar nomination and win for a role that took up about three minutes of screentime, or something. Basically, she was in one scene at the end of a long movie, and walked away with an Oscar.
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.