There is no official consensus or definition for the length of a “short.”
Any film shorter than 50 minutes qualifies as a short film for the Sundance Film Festival, while any film with a run-time under 40 minutes is eligible for the shorts categories at the Academy Awards.
The longest of the ten movies nominated for either best live-action or best animated short at the 83rd Academy Awards is The Gruffalo, at 27 minutes.