stundenlang
for hours
Continuing over a period of several hours.
Wir haben stundenlang über seine Zukunftspläne diskutiert.
We discussed his plans for the future for hours.

for hours
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for hours
Continuing over a period of several hours.
Wir haben stundenlang über seine Zukunftspläne diskutiert.
We discussed his plans for the future for hours.

The word is simply built from 'Stunden' (hours, plural of Stunde) and 'lang' (long). It literally means 'for the duration of hours.'
Die Reparatur des Autos dauerte stundenlang.
The repair of the car took hours.
The structure [unit of time] + 'lang' is very productive. Following the same pattern, you can also form 'tagelang' (for days), 'wochenlang' (for weeks), 'monatelang' (for months), and 'jahrelang' (for years).
Nach dem Sturm waren wir tagelang ohne Strom.
After the storm, we were without power for days.
As an adverb, 'stundenlang' (for hours) usually appears in the middle of the sentence to describe the duration of an action. It can also appear at the beginning of the sentence for emphasis; then the verb follows in second position.
Die Kinder haben stundenlang im Garten gespielt.
The children played in the garden for hours.
brief / temporary
'stundenlang' and 'kurzzeitig' both describe how long something lasts. But they are clearly opposites. 'stundenlang' means that something lasts for several hours and is often perceived as a long time. 'kurzzeitig' means that something happens or applies only for a short time. 'stundenlang' is usually used as an adverb with actions, while 'kurzzeitig' is often used as an adjective for states or changes.

Maria wartet heute stundenlang in der Werkstatt auf ihr Auto.
Maria waits for hours at the repair shop today for her car.
Wegen einer Störung ist die Internetverbindung kurzzeitig weg.
Because of a fault, the internet connection is briefly down.