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Lengthy. This word is of comparatively recent origin, and, though, it is said to be an Americanism, it is a good deal used in England. The most careful writers, however, both here and elsewhere, prefer the word long: "a long discussion," "a long discourse," etc.

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lengthy is an Americanism long established in Britain, sometimes used as a jocular or stylish synonym for long but more commonly and usefully as implying tediousness as well as length.

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lengthy. Throughout the 19th century, many BrE speakers considered this word and ugly Americanism. But it is now standard throughout the English-speaking world. The only restriction is that it should refer to books, talks, or arguments, with the implication of tedium. It shouldn't refer to physical distances, as here: "Women longshoremen sometimes have to drive lengthy [read long] distances to find one [a toilet] or to locate someone with a key." Virginian-Pilot & Ledger Star (Norfolk), 10 Apr. 1997, at 6.

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Lengthy started its career as a usage issue in the late 1700s, when British critics began to attack it as an Americanism. The first record of its use is from about a hundred years earlier. Among the 18th-century American writers who used it were Thomas Jefferson, John Adams, Alexander Hamilton, George Washington, and Benjamin Franklin.

[...]

[A] British nobleman named Lord Harrowby defended lengthy as early as 1818, finding that it "imported what was tedious as well as long, an idea that no other English word seemed to convey as well." Lengthy became so firmly established in British usage during the 19th century that [Albert] Mathews [in The Dial in 1898] felt safe in concluding that "the controversy may be said long ago to have ended."

As it turns out, Mathew's conclusion was a bit premature. Lengthy has continued to appear sporadically in books on usage (most of them American) throughout the 20th century. [Ambrose] Bierce 1909 thought that it was "no better than breadthy, or thicknessy," but later commentators have been more tolerant.

[...]

The upshot of this lengthy discussion (which you may find either tedious or comprehensive) is that lengthy is a venerable synonym of long which has been used by excellent writers for about three centuries. You need not hesitate to use it yourself.