For a long time; at great length — often used as 'naga-naga to' to indicate something continues or is said at length (sometimes tediously).
長々と説明してしまい、みんな退屈そうだった。
I ended up explaining at length and everyone looked bored.
長々お待たせして申し訳ありませんでした。
I'm sorry to have kept you waiting for so long.
彼は会議で長々と意見を述べた。
He gave a long-winded opinion at the meeting.
Extending far or stretching out; lying at full length or similar senses of extended length.
午後はソファに長々と寝そべって本を読んだ。
In the afternoon I lay at full length on the sofa and read a book.
山道が長々と谷の方へ続いている。
The mountain path stretches a long way down into the valley.