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.
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.