Collective nouns may take either singular or plural verbs, depending on whether the word refers to the group as a unit or to its members as individuals. In American English, most nouns naming a group regarded as a unit are treated as singular. (See also 7.8.5, Grammar, Subject-Verb Agreement, Collective Nouns.)
Fifty percent of my time is spent on administration.
Fifty percent of all physicians do not exercise regularly.
The audience was enthralled.
This gathering is becoming noisy.
For a unit of measure, use a singular verb.
At noon today the jury delivers its verdict.
Five milliliters was injected.
Two weeks of symptoms is common.