There's a commonly repeated rule about fewer and less. It goes like this: fewer is used to refer to number among things that are counted, as in "fewer choices" and "fewer problems"; less is used to refer to quantity or amount among things that are measured, as in "less time" and "less effort."
Use less with singular nouns; fewer, with plural nouns. But if you’re using a plural noun to measure a chunk or amount of money or time, then you should use less (less than five years, less than 2,500 dollars).