A considerable/long period of time; lasting several hours or more. Can be used as a noun or adverb (e.g., 'for a long time').
長時間の会議で疲れがたまった。
I got tired from the long meeting.
このバッテリーは長時間動作する。
This battery operates for a long time.
長時間歩いたので休憩を取った。
We took a break because we had been walking for a long time.