This is the 1429th most frequent Russian word.
долго
Long; for a long time.
The word 'долго' is used here as an adverb meaning 'long' or 'for a long time,' describing the duration of thinking.
Я долго думал над этим вопросом.
I thought about this question for a long time.
In this sentence, 'долго' functions as an adverb indicating a longer-than-expected duration for responding.
Почему ты так долго отвечал?
Why did you take so long to respond?
Here, 'долгой' is an adjective derived from 'долго,' describing 'речь' (speech) as overly lengthy in duration.