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Time, distance, weight are treated as singular, like "3 hours is too long." Then, in this instance,

How many hours of sleep is /are needed to keep fit?"

Is "is" correct?

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Part-1 of the post, the presumption, is invalid. Part-2, for the example sentence, for the question "Is is correct", the answer would be 'Yes', but not deriving from the presumption. – Kris Feb 14 '13 at 8:50
    
"3 hours is a long time" = "the period is a long time". "How many hours of sleep are needed" = "how many periods of sleep are needed". – Max Williams Jun 13 at 7:45

When time, distance, money, and weight are considered as collective units of measure, they are considered singular because they're collective:

Three hours is a long time.
Fifty kilometers is too far to walk.
Five thousand dollars is too much for that ratty old fur coat.
Four hundred kilograms is more than my extended family of five weighs. (The final verb might be "weigh" in British English. I can't say for sure.)

However, when the units are considered as separate entities, they're considered plural:

"Many hours are lost in arguing over even obvious errors." (Kris's example)
Those last five kilometers were killers, each for a different reason: crocodiles, mosquitoes, rattlesnakes, robbers, and army ants.
There are fifty silver dollars in the piggy bank. (There are 50 coins....)
The last three kilograms were lost at a rate of one per week instead of one per day. (Separate units of weight)

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1  
I could guess collective means whole and separate means some part of the whole..? Thanks for big help! – anox Feb 14 '13 at 9:47

Three hours is a long time.

is similar to

A three-hour duration is a long time.

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protected by Mari-Lou A Jun 13 at 7:14

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