Under the general rule, fewer should be used for plural nouns and things that can actually be counted while less is used for collective nouns, mass nouns, or abstract nouns. The exceptions to this rule involve common phrases that allow less to become acceptable in many situations (e.g., less than is common for time, distance, and money).
When referring to time or money, the word less is appropriate, even though time (days, weeks, etc.) and money (dollars) can be counted. You would use less because you’re referring to an amount of time or an amount of money. The expression less than 10 dollars means “any amount of $9.99 or less.” If you wrote fewer than 10 dollars, you would refer to 9 or 8 or 7, etc.
Most grocery stores make a blunder with their signs on express lanes: “15 items or less.” Safeway gets it right: “15 items or fewer.”
Example: There were fewer than a dozen people in the elevator, and we were stuck for less than five minutes, but the experience was still less than enjoyable.