This is the 12773rd most frequent German word.
wochenlange
"Wochenlange" means "weeks-long" or "lasting for weeks."
In this sentence, "wochenlangen" is used as an adjective to describe "Verhandlungen" (negotiations), indicating something that lasted for several weeks.
Nach wochenlangen Verhandlungen erreichten sie eine Einigung.
After weeks-long negotiations, they reached an agreement.
Here, "wochenlange" modifies "Reise" (journey), again showing the event lasted over the span of several weeks.
Die wochenlange Reise ermüdete die ganze Familie.
The weeks-long journey exhausted the whole family.
In this example, "wochenlangen" describes "Pause" (break), emphasizing the duration of the break.