How long did it take to recover from the collapse of the Roman Empire?

This is often overstated.  The parts of the Empire that collapsed into the dark ages were the parts that had never been civilized (as in, having a well-established and indigenous culture of cities, settled life, and state-level social organization).  Minus Italy, the parts of the Empire that had long traditions of civilization survived and thrived in the post-Roman period.  And Italy had been so destroyed by the Roman slave-economy that it hardly even counted as civilized by the time the empire ended, and what veneer of civilization that remained was destroyed in the Byzantine re-conquest.