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

and as likely as fewer to modify periods of time.