"He sleeps two-thirds of the time."
Translation:Il dort les deux tiers du temps.
I was refused this answer when I omitted the definite article. Other sources say that the def art is optional here, and that my answer should have been accepted.
Statistically, it is safer for you to go for the rule: nouns need determiners in French.
You can omit it in this sentence, but generally, it's to specify which "two thirds" we're referring. For instance, les deux tiers du temps, je dors, et l'autre tiers, je travaille