The word “fewer” is correct when describing an amount of discrete items, or items which may be individually counted, where the number of items has decreased from a previous measurement. “Less” is used when what is being measured is not inherently divisible but has decreased in general. Examples:

Fewer hours equals less time.
Fewer dollars equals less money.
Fewer horses equals less power.
Fewer grains equals less sand.
Fewer gallons equals less water.

Can you think of other examples?