This is the 2926th most frequent Polish word.
najdłużej
"Najdłużej" means "the longest" (in terms of time) in English.
Here, 'najdłużej' is used as an adverb describing the duration of staying compared to others.
Zostałem tam najdłużej ze wszystkich.
I stayed there the longest of all.
In this sentence, 'najdłużej' indicates the time duration required by a specific action.
Która droga prowadzi najdłużej do celu?
Which road takes the longest to reach the destination?
The word 'najdłużej' here qualifies as an adverb indicating the duration within a descriptive context.