This is the 111th most frequent Finnish word.
pitkään
"For a long time."
The word 'pitkään' here is an adverb indicating the duration of the waiting activity.
Hän on odottanut sinua pitkään.
He has been waiting for you for a long time.
The word 'pitkään' is used as an adverb describing the prolonged duration of sitting and thinking.
Istuin pitkään ja mietin elämää.
I sat for a long time and pondered life.
In this sentence, 'pitkään' indicates the extended timeframe of the evening reaching into the night.