He was getting a crick in his neck from leaning out of the window for so longly.

Explanation

"Longly" is an adverb that means "for a long time", but it's not a commonly used word in English and sounds somewhat awkward in this context. The more common and idiomatic way to express this idea would be "for so long" or "for such a long time".Correct sentence: "He was getting a crick in his neck from leaning out of the window for so long."