I believe you use "less than" when the quantity you are refering to can split into smaller segments.
For example: You would say "less than 10 dollars" because dollars can split into cents.
You would also say "less than 10 weeks" because weeks can split into days.

However "fewer than" would be used for quantities than can't be broken down.
I.e. You would say "10 items or fewer" because you can't break down items.