The German terms stundenlang, monatelang, tagelang, wochenlang, and jahrelang are used to express durations of time in a descriptive manner. 

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Stundenlang translates to "for hours" or even "for hours on end," indicating an extended period spent on an activity and often emphasizing the intensity or monotony of the experience. This word can either be used as an adverb or as an adjective, in which case it needs to be altered to reflect the noun that follows. 

Stundenlang ritt er umher, um den Weg nach Hause zu finden.

For hours, he rode around searching for the way home.

Caption 19, Märchen - Sagenhaft: Hans mein Igel

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Und ich hätte wirklich stundenlang dir zuhören können.

And I really could have listened to you for hours.

Caption 42, The Voice of Germany: Isabel Nolte singt „Als ich fortging“

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Zurzeit muss man entweder eine stundenlange Zugfahrt in Kauf nehmen oder man fliegt.

Currently, you either have to put up with an hours-long train ride, or you fly.

Captions 22-23, DW-Nachrichten: Der Traum vom Hyperloop

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The word tagelang highlights a continuous action or state over several days, and is also often used in narratives to depict persistence or exhaustion.

Tagelang fuhren sie bergauf, bergab über die holprigen Straßen.

For days, they rode uphill, downhill over the bumpy roads.

Caption 44, Märchen - Sagenhaft: König Drosselbart

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Ich könnte tagelang von dir erzählen.

I could talk about you for days.

Caption 16, Xavier Naidoo Ich kenne nichts (das so schön ist wie du)

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The word wochenlang is also used as an adverb of time or an adjective that describes something that lasts weeks.

Das "Face" ist wochenlang beobachtet und perfekt gecheckt worden.

The face has been studied and checked perfectly for weeks.

Caption 15, Wintersport: Engadin Snow 2009

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Man kann auch von Menschen, die wochenlang unterwegs waren, die Deutschland als ihr Ziel ausgeben, etwas Geduld erwarten.

You can also expect, from people who were traveling for weeks, who identify Germany as their goal, some patience.

Captions 10-11, rheinmaintv: De Maizière führt Gespräch über potenzielle Sicherheitsgefahr

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In these last two examples, you can see monatelang used as an adverb and jahrelang used as an adjective:

Du kannst ja jetzt auch nicht monatelang im Prinzip ständig am Limit trainieren, du brauchst auch wieder eine Regeneration und so.

You also can't—for months on end now—be pushing yourself to the limit when training, simply as a matter of principle. You also need a period of regeneration, and so on.

Captions 52-54, Angelique Kerber: Ihre tennisfreie Zeit

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Da ist Technik dabei und die jahrelange Erfahrung und Tricks.

Technical [assistance] is there and the long years of experience and tricks of the trade.

Caption 55, Selbst versucht: Gepäckabfertigung bei Fraport

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Further Learning
You can find more examples of these words used in context on Yabla German. Also, try writing your own sentences.