"vacant hour" is a correct and usable phrase in written English.
It means a period of time in which there are no scheduled activities or obligations. Example: I have a few vacant hours before my afternoon meeting, so I'm going to use that time to catch up on some reading.
Exact(1)
He is apt to pronounce that there are "innocent delights enough to fill with joy every vacant hour" in order to persuade a friend to give up womanizing which the friend does.
Similar(58)
It was when she quit those teams in her midteens, her father said, that she stumbled into trouble, filling the vacant hours with drugs and delinquency.
Still, Mr. Ostrowski keeps a full schedule, though he says it has been something of a task to fill up the vacant hours.
Thus, by mutual good offices, each seemed to console the vacant hours of the other: so that Milton, when he puts the following sentiment in the mouth of Adam, seems to be somewhat mistaken: Much less can bird with beast, or fish with fowl, So well converse, nor with the ox the ape.
Suggest them ways how to spend their vacant hours, better yet by encouraging them to spend it by reading a good science fiction book while munching on their food.
The replacement - which even had a skeleton fish tail - was placed in the same position as the Little Mermaid and sat in her vacant spot for two hours, to the delight of tourists.
At 1700 hours, Germans entered the vacant Villy Est casemate, occupying Villy Ouest an hour later.
A COMMON frustration with London's subsidised cycle-hire scheme is that it can be insanely hard to find a bike in rush hour, or a vacant docking slot at the end of a ride.
He was the seventh participant in "One & Other," a grand project that is meant to stretch the boundaries of sculpture by placing 2,400 people on the square's usually vacant fourth plinth, for an hour apiece, from now through Oct. 6.
It was never much of a surprise that Boris was linked with the vacant Top Gear job within hours of his Tory leadership bid crashing.
Jason Gillespie will hold talks with England director Andrew Strauss in the next 24 hours over the vacant head coach's position.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com