Use fewer with objects that can be counted one-by-one.
Use less with qualities or quantities that cannot be individually counted.
Incorrect: There were less days below freezing last winter.
Correct: There were fewer days below freezing last winter.
(Days can be counted.)
Correct: I drank less water than she did.
(Water cannot be counted individually here.)
When referring to time or money, less is normally used even with numbers. Specific units of time or money use fewer only in cases where individual items are referred to.
Examples: I have less than an hour to do this work.
I have less time to do this work.
I have less money than I need.
I have less than twenty dollars.
He worked fewer hours than I did.
The only occasion in which you might say, "I have fewer than twenty dollars," would be when you were talking about specific dollar bills or coins, such as "I have fewer than twenty silver dollars in my collection."
See also Irregular comparisons.