"frightening lengths" is correct and usable in written English.
It is an idiom meaning to go to great extremes, and is typically used in a negative way. For example, "The politician went to frightening lengths to discredit his opponents."
Ed Begley won the best supporting actor Oscar as Boss Finley, a local politician who will go to frightening lengths to keep his oddly named daughter, Heavenly (Shirley Knight), away from Chance.
Here's a taste from the second preface Nietzsche wrote to it: "The unconscious disguise of physiological needs under the cloaks of the objective, ideal, purely spiritual goes to frightening lengths – and often I have asked myself whether, taking a large view, philosophy has not been merely an interpretation of the body, and a misunderstanding of the body".
They were brazen, persistent and "ruthless" — and "willing to go to frightening lengths" to extort money, said Bruce Foucart, an assistant director at the United States Immigration and Customs Enforcement agency.
It's frightening, the lengths he'll go to make a point".
Aruna is not alone - her story is repeated with a frightening regularity across the length and breadth of the country.
Nobody knew the piece very well then, and the seemingly random accents and erratic phrase lengths were a constant, disruptive, frightening shock.
The lengths she will go to in order to protect her family can be frightening.
The Mini S is fairly frightening to ride at times, there's something a bit more comforting about the extra length and flex of the longboards.
Set in 1976 on a "wild and useless length of English coastline" somewhere in the north-west, The Loney's landscape is both timeless and frightening.
