This is the 5113th most frequent Russian word.
надолго
For a long time; long-term.
Here, 'надолго' emphasizes the duration of the described activity, indicating that the vacation will last for an extended period of time.
Он уехал в отпуск надолго.
He went on vacation for a long time.
In this sentence, although negated ('ненадолго'), it still refers to the extent or duration of time, now suggesting a brief duration.
Ты смог бы оставить мне ключи ненадолго?
Could you leave me the keys for a short while?
'Надолго' is used metaphorically here, indicating that the memory of the event will persist for an extended time span in one's mind.