The graduation ceremony lasted only two hours.
what part of the sentence is lasted only two hours

The answer to your question would be that "lasted only two hours" is the predicate of the sentence.

The predicate of a sentence is the part that modifies the subject in some way. Since the subject is the person, thing, or place that the sentence is about, the predicate must contain a verb explaining what the subject does and can also include a modifier.

Explanation:

He hadn't made any attempts to escape.

They had taken the boy to the doctor.

The cat had killed the mouse.

The dacoits had looted the shop.

They had accepted the invitation.