For the Qualifying Child dependent rule, you must have been full time for parts of calendar months. Graduating in May, even May 1st, means you usually meet this rule.
You don't say why you're asking. It's usually about being claimed as a dependent. See additional reply for a discussion on that.
That particularly important this year, with stimulus $ on the table.
Under the CARES Act, if you are claimed, or qualify to be claimed, as a dependent on someone else’s return you cannot receive a stimulus check, in 2020. If you qualified as a dependent for 2019, but will not be for 2020, you will most likely get it in 2021, when you file a 2020 tax return.
