I'd use "is" in the first example because it usually implies "A period/interval of 24 hours is enough". I'd accept both in the second example. (Having) two kids is enough.
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AnonymousWhen talking about hours at work is it, "twenty hours is enough, or twenty hours are enough?"Use the singular if you are measuring something. Use the plural if you are counting.
When talking about kids is it, "two kids is enough, or two kids are enough?"