What do the words long I stood indicate?

The phrase "long I stood" means that the speaker stood, thinking or pondering, in one place. This phrase is part of a line from Edgar Allan Poe's 1845 poem The Raven.

Deep into that darkness peering, long I stood there wondering, fearing,

Doubting, dreaming dreams no mortal ever dared to dream before;

But the silence was unbroken, and the stillness gave no token,

And the only Word there spoken was the whispered Word, "Lenore?"