The traditional view is that less applies to matters of degree, value, or amount and modifies collective nouns, mass nouns, or nouns denoting an abstract whole while fewer applies to matters of number and modifies plural nouns. Less has been used to modify plural nouns since the days of King Alfred and the usage, though roundly decried, appears to be increasing. Less is more likely than fewer to modify plural nouns when distances, sums of money, and a few fixed phrases are involved <less than 100 miles><an investment of less than $2000><in 25 words or less> and as likely as fewer to modify periods of time <in less (or fewer) than four hours>.
Examples of LESS
We need less talk and more work!
She finished in less time than I did.
Less detail is sometimes better than more detail.
8 times 2 is less than 6 times 3.
We made it there in less than six hours.
The whole procedure takes less than five minutes.
The illness affects less than one percent of the population.
Origin of LESS
Middle English, partly from Old English lǣs, adverb & noun; partly from lǣssa, adjective; akin to OldFrisian lēs less