Fewer for countable nouns, less for uncountable (although many people use less for everything)

I must say more. Imagine instead that the sentence were "The recipe requires less/fewer than 5 apples". Which is correct? "Apples" are discrete objects. You can count them: one apple, two apples, three apples, etc. Those who prefer to classify nouns as being "countable" or "not countable" (note: I am not such a person) are likely to recommend saying "fewer than". But apples CAN be subdivided. Perhaps the recipe works better with 4 1/2 apples. Since that is a possibility it makes sense to say "less than 5". In fact, that choice is really more logical because apples really are infinitely subdivisible. I would argue that both are correct. "Less" makes sense because it corresponds to the truth. "Fewer" makes sense because by using the plural "apples" you indicate that you are thinking of apples as discrete objects. That one can argue both ways with so many nouns is the reason why I prefer to avoid the categories "countable" and "not countable" altogether. It is important for students to be aware of the distinction between discrete and continuous phenomena. It is important to know the distinct ways of describing them (such as "less" vs. "fewer"). However, it is a mistake to try to divide words between those two categories. One more thing. The choice of words "countable" and "uncountable" is most unfortunate. "Discrete" and "continuous" would be more descriptive terms.

Both are perfect. The way we think about time is flexible. Since time is infinitely divisible, we can think of it as a continuous phenomenon so "less" makes sense. On the other hand, we can think of "seconds" as a discrete phenomenon so "fewer" makes sense too. There is no reason whatsoever to prefer one choice over the other.

less is better，I think